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Abstract:

Provided herein are systems and methods for operating a traveling wave
linear accelerator to generate stable electron beams at two or more
different intensities by varying the number of electrons injected into
the accelerator structure during each pulse by varying the width of the
beam pulse, i.e., pulse width. The electron beams may be used to generate
x-rays having selected doses and energies, which may be used for cargo
scanning or radiotherapy applications.

Claims:

1. A traveling wave linear accelerator for generating a plurality of dose
rates and energies of electrons, the traveling wave linear accelerator
comprising: an electron gun modulator configured to adjust a pulse width
and a beam injection time of a beam of electrons from an electron gun; a
frequency controller configured to determine a frequency of a signal to
be generated; and an intensity controller operatively associated with the
electron gun modulator and the frequency controller, the intensity
controller configured to receive a plurality of intensity/energy
adjustment commands and to respectively determine a pulse width, a beam
injection time, and a frequency adjustment factor based on each
intensity/energy adjustment command to provide a respective output dose
rate and energy of electrons; wherein, for each intensity/energy
adjustment command, the electron gun modulator receives the determined
pulse width and the determined beam injection time and adjusts the pulse
width and the beam injection time of the beam of electrons and the
frequency controller receives the frequency adjustment factor and adjusts
the frequency of the signal such that the traveling wave linear
accelerator generates electrons having the respective output dose rate
and energy.

2. The traveling wave linear accelerator of claim 1, further comprising:
an x-ray target configured to generate x-rays responsive to irradiation
with electrons, the x-rays irradiating a cargo container; and a detector
configured to detect x-rays transmitted through the container.

3. The traveling wave linear accelerator of claim 2, further comprising a
control unit operatively associated with the detector and with the
intensity controller, the control unit being configured: to send a first
intensity/energy adjustment command to cause the intensity controller to
determine a first pulse width, a first beam injection time, and a first
frequency adjustment factor to provide a first output dose rate and first
energy of a first set of electrons; to determine a percent transmission
of a first set of x-rays through the container based on an output of the
detector, the first set of x-rays being generated by the first set of
electrons; and if the percent transmission is below a predetermined
threshold, to send a second intensity/energy adjustment command to cause
the intensity controller to determine a second pulse width, a second beam
injection time, and a second frequency adjustment factor to provide a
second output dose rate and second energy of a second set of electrons.

4. The traveling wave linear accelerator of claim 3, wherein the second
energy is higher than the first energy.

5. The traveling wave linear accelerator of claim 4, wherein the
intensity controller is configured to select the second output dose rate
of the second set of electrons such that a dose of the first set of
x-rays is about the same as a dose of a second set of x-rays generated by
the second set of electrons.

6. The traveling wave linear accelerator of claim 3, wherein the control
unit is configured: to determine a percent transmission of a second set
of x-rays through the container based on an output of the detector, the
second set of x-rays being generated by the second set of electrons; and
if the percent transmission is below a predetermined threshold, to send a
third intensity/energy adjustment command to cause the intensity
controller to determine a third pulse width, a third beam injection time,
and a third frequency adjustment factor to provide a third output dose
rate and third energy of a third set of electrons.

7. The traveling wave linear accelerator of claim 6, wherein the third
energy is higher than the second energy.

8. The traveling wave linear accelerator of claim 7, wherein the
intensity controller is configured to select the third output dose rate
of the third set of electrons such that a dose of the third set of x-rays
is about the same as a dose of a third set of x-rays generated by the
third set of electrons.

9. The traveling wave linear accelerator of claim 6, wherein the third
energy is lower than the second energy and wherein the intensity
controller is configured to select the third output dose rate of the
third set of electrons such that a dose of the third set of x-rays is
greater than a dose of a first set of x-rays generated by the first set
of electrons.

10. The traveling wave linear accelerator of claim 2, further comprising
a control unit operatively associated with the detector and with the
intensity controller, the control unit being configured: to send a first
intensity/energy adjustment command to cause the intensity controller to
determine a first pulse width, a first beam injection time, and a first
frequency adjustment factor to provide a first output dose rate and first
energy of a first set of electrons; to determine a percent transmission
of a first set of x-rays through the container based on an output of the
detector, the first set of x-rays being generated by the first set of
electrons; and if the percent transmission is above a predetermined
threshold, to send a second intensity/energy adjustment command to cause
the intensity controller to determine a second pulse width, a second beam
injection time, and a second frequency adjustment factor to provide a
second output dose rate and a second energy of a second set of electrons.

11. The traveling wave linear accelerator of claim 10, wherein the
intensity controller is configured to select the second output dose rate
of the second set of electrons such that a dose of the second set of
x-rays generated by the second set of electrons is less than a dose of
the first set of x-rays.

12. The traveling wave linear accelerator of claim 1, further comprising:
an x-ray target configured to generate x-rays responsive to irradiation
with electrons from the traveling wave linear accelerator, the x-rays
being configured to irradiate a tumor volume; and a robotic arm on which
the x-ray target and the linear accelerator are mounted and configured to
adjust an angle at which the x-rays irradiate the tumor volume.

13. The traveling wave linear accelerator of claim 12, further comprising
a control unit operatively associated with the robotic arm and with the
intensity controller, the control unit being configured: to send a first
intensity/energy adjustment command to cause the intensity controller to
determine a first pulse width, a first beam injection time, and a first
frequency adjustment factor to provide a first output dose rate and a
first energy of a first set of electrons; to send a first position
command to the robotic arm to cause the robotic arm to adjust the angle
to irradiate a first portion of the tumor volume with x-rays generated by
the first set of electrons; to send a second intensity/energy adjustment
command to cause the intensity controller to determine a second pulse
width, a second beam injection time, and a second frequency adjustment
factor to provide a second output dose rate and a second energy of a
second set of electrons; and to send a second position command to the
robotic arm to cause the robotic arm to adjust the angle to irradiate a
second portion of the tumor volume with x-rays generated by the second
set of electrons.

14. The traveling wave linear accelerator of claim 13, wherein the second
energy is higher than the first energy.

15. The traveling wave linear accelerator of claim 14, wherein the second
tumor volume is deeper than the first tissue volume.

16. The traveling wave linear accelerator of claim 16, wherein the first
tissue volume and the second tissue volume receive about the same doses
of x-rays as one another.

17. A method for generating a plurality of dose rates and energies of
electrons using a traveling wave linear accelerator, the traveling wave
linear accelerator comprising an electron gun modulator configured to
adjust a pulse width and a beam injection time of a beam of electrons
from an electron gun and a frequency controller configured to adjust a
frequency of a signal to be generated, the method comprising: receiving
at an intensity controller a plurality of intensity/energy adjustment
commands and respectively determining a pulse width, a beam injection
time, and a frequency adjustment factor based on each intensity/energy
adjustment command to provide a respective dose rate and energy of
electrons; and for each intensity adjustment command: adjusting the pulse
width and beam injection time of electrons from the electron gun at the
electron gun modulator using the determined pulse width and the
determined beam injection time; determining the frequency of the signal
to be generated at the frequency controller based on the frequency
adjustment factor; and generating electrons having the respective output
dose rate and energy using the traveling wave linear accelerator.

18. The method of claim 17, further comprising: sending a first
intensity/energy adjustment command to cause the intensity controller to
determine a first pulse width, a first beam injection time, and a first
frequency adjustment factor to provide a first output dose rate and first
energy of a first set of electrons; generating x-rays with the first set
of electrons; irradiating a cargo container with the x-rays; determining
a percent transmission of a first set of x-rays through the container
based on an output of the detector; and if the percent transmission is
below a predetermined threshold, sending a second intensity/energy
adjustment command to cause the intensity controller to determine a
second pulse width, a second beam injection time, and a second frequency
adjustment factor to provide a second output dose rate and second energy
of a second set of electrons.

19. The method of claim 17, further comprising: sending a first
intensity/energy adjustment command to cause the intensity controller to
determine a first pulse width, a first beam injection time, and a first
frequency adjustment factor to provide a first output dose rate and a
first energy of a first set of electrons; sending a first position
command to a robotic arm on which the linear accelerator and an x-ray
target are mounted to cause the robotic arm to adjust an angle to
irradiate a first portion of a tumor volume with x-rays generated by the
first set of electrons; sending a second intensity/energy adjustment
command to cause the intensity controller to determine a second pulse
width, a second beam injection time, and a second frequency adjustment
factor to provide a second output dose rate and a second energy of a
second set of electrons; and sending a second position command to the
robotic arm to cause the robotic arm to adjust the angle to irradiate a
second portion of the tumor volume with x-rays generated by the second
set of electrons.

Description:

1. CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 12/976,787, filed Dec. 22, 2010 and entitled
"Traveling Wave Linear Accelerator Based X-Ray Source Using Pulse Width
to Modulate Pulse-to-Pulse Dosage," which claims the benefit of U.S.
Provisional Application No. 61/389,149, filed Oct. 1, 2010, the entire
content of which is incorporated herein by reference.

2. TECHNICAL FIELD

[0002] The invention relates to systems and methods for use in cargo
scanning and radiotherapy based on generating x-rays with modulated
pulse-to-pulse dosage using a traveling wave linear accelerator by
varying pulse width.

3. BACKGROUND OF THE INVENTION

[0003] Linear accelerators (LINACs) are useful tools for industrial
applications, such as radiography, cargo inspection and food
sterilization, and medical applications, such as radiation therapy and
imaging. In some of these applications, beams of electrons accelerated by
the LINAC are directed at the sample or object of interest for analysis
or for performing a procedure. However, in many of these applications, it
can be preferable to use x-rays to perform the analysis or procedure.
These x-rays may be generated by directing the electron beams from the
LINAC at an x-ray emitting target.

[0004] A cargo inspection device that uses x-rays generated from a LINAC
is useful during non-intrusive inspection of cargo because of the high
energy output (and therefore greater penetration) that it provides. As a
result, large quantities of containers may inspected more accurately
without requiring inspectors to open the containers.

[0005] Typically, the LINACs used in cargo inspection systems are
configured to produce a single energy x-ray beam. A detector receives the
single energy x-ray beam that has penetrated the shipping container
without being absorbed or scattered, and produces an image of the
contents of the shipping container. The image may be displayed to an
inspector who can perform visual inspection of the contents. The
inspector may observe contents in the container that require further
analysis. It has been suggested to vary the x-ray dosage, i.e.,
intensity, to further inspect dense cargo. It would be desirable to
provide a LINAC based x-ray source configured to modulate pulse-to-pulse
intensity while outputting energy stable electron beams from the LINAC.

[0006] Other previously-known cargo inspection devices use dual energy
LINACs that are configured to emit two different energy level x-ray
beams. With a dual energy x-ray inspection system, materials can be
discriminated radiographically by alternately irradiating an object with
x-ray beams of two different energies. Dual energy x-ray inspection
systems can determine a material's mass absorption coefficient, and
therefore the effective atomic (Z) number of the material.
Differentiation is achieved by comparing the attenuation ratio obtained
from irradiating the container with low-energy x-rays to the attenuation
ratio obtained from irradiating the container with high-energy x-rays.
Discrimination is possible because different materials have different
degrees of attenuation for high-energy x-rays and low-energy x-rays, and
that allows identification of low-Z-number materials (such as but not
limited to organic materials), medium-Z-number materials (such as but not
limited to transition metals), and high-Z-number materials (such as but
not limited to radioactive materials) in the container. Such systems can
therefore provide an image of the cargo contents and identify the
materials that comprise the cargo contents.

[0007] The ability of dual energy x-ray inspection systems to detect the Z
number of materials being scanned enables such inspection systems to
automatically detect the different materials in a container, including
radioactive materials and contraband such as but not limited to cocaine
and marijuana. However, conventional dual energy x-ray inspection systems
use a standing wave LINAC that is vulnerable to frequency and power
jitter and temperature fluctuations, causing the beam energy from the
linear accelerator to be unstable when operated to accelerate electrons
to a low energy. The energy jitter and fluctuations can create image
artifacts, which cause an improper Z number of a scanned material to be
identified. This can cause false positives (in which a targeted material
is identified even though no targeted material is present) and false
negatives (in which a targeted material is not identified even though
targeted material is present).

[0008] Like single energy x-ray inspection systems, dual energy x-ray
inspection systems may produce an image of the contents of a shipping
container that may be displayed to an inspector who can perform visual
inspection of the contents. The inspector may observe contents in the
container that require further analysis. Accordingly, it would be
desirable to provide a dual energy LINAC based x-ray inspection system
configured to modulate pulse-to-pulse intensity to increase an
inspector's ability to accurately investigate cargo.

[0009] Radiotherapy applications also may employ single or dual energy
x-rays in irradiating a tumor volume so as to cause necrosis of the
volume. It would be desirable to modulate pulse-to-pulse intensity to
enhance the treating physician's ability to more homogeneously irradiate
the tumor volume.

4. SUMMARY OF THE INVENTION

[0010] The present invention provides a traveling wave linear accelerator
(TW LINAC) based x-ray source configured to modulate pulse-to-pulse
intensity while outputting energy stable electron beams of varying
energies. The TW LINAC may be configured for use in both cargo scanning
and radiotherapy applications.

[0011] The TW LINAC of the present invention includes an electron gun
modulator configured to adjust a pulse width and a beam injection time of
a beam of electrons from an electron gun, and a frequency controller
configured to determine a frequency of a signal to be generated. The TW
LINAC further includes an intensity controller operatively associated
with the electron gun modulator and the frequency controller. The
intensity controller is configured to receive a plurality of
intensity/energy adjustment commands and to respectively determine a
pulse width, a beam injection time, and a frequency adjustment factor
based on each energy/intensity adjustment command to provide a respective
output dose rate and energy of electrons. For each intensity/energy
adjustment command, the intensity controller transmits the determined
pulse width and the determined beam injection time to the electron gun
modulator so that the modulator commands an electron gun to adjust the
outputted pulse width and beam injection timing of electrons and the
frequency controller receives the frequency adjustment factor and adjusts
the frequency of the signal such that the traveling wave linear
accelerator generates electrons having the respective output dose rate
and energy.

[0012] In embodiments suitable for use in cargo scanning applications, the
traveling wave linear accelerator further includes an x-ray target
configured to generate x-rays responsive to irradiation with electrons,
the x-rays irradiating a cargo container; and a detector configured to
detect x-rays transmitted through the container. A control unit may be
operatively associated with the detector and with the intensity
controller. The control unit may be configured to send a first intensity
adjustment command to cause the intensity controller to determine a first
pulse width, a first beam injection time, and a first frequency
adjustment factor to provide a first output dose rate and first energy of
a first set of electrons. The control unit further may be configured to
determine a percent transmission of a first set of x-rays through the
container based on an output of the detector, the first set of x-rays
being generated by the first set of electrons. The control unit also may
be configured to send a second intensity/energy adjustment command to
cause the intensity controller to determine a second pulse width, a
second beam injection time, and a second frequency adjustment factor to
provide a second output dose rate and second energy of a second set of
electrons, for example if the percent transmission is below a
predetermined threshold.

[0013] The second energy may be higher than the first energy. The
intensity controller may be configured to select the second output dose
rate of the second set of electrons such that a dose of the second set of
x-rays generated by the second set of electrons is about the same as that
of the first set of x-rays.

[0014] The control unit may be configured to determine a percent
transmission of a second set of x-rays through the container based on an
output of the detector, the second set of x-rays being generated by the
second set of electrons; and, if the percent transmission is below a
predetermined threshold, to send a third intensity/energy adjustment
command to cause the intensity controller to determine a third pulse
width, a third beam injection time, and a third frequency adjustment
factor to provide a third output dose rate and third energy of a third
set of electrons. The third energy may be higher than the second energy.
The intensity controller may be configured to select the third output
dose rate of the third set of electrons such that a dose of a third set
of x-rays generated by the third set of electrons is about the same as
that of the second set of x-rays. Alternatively, the third energy may be
lower than the second energy and the intensity controller may be
configured to select the third output dose rate of the third set of
electrons such that a dose of the third set of x-rays is greater than a
dose of a first set of x-rays generated by the first set of electrons.

[0015] Some embodiments may include a control unit operatively associated
with the detector and with the intensity controller, the control unit
being configured to send a first intensity/energy adjustment command to
cause the intensity controller to determine a first pulse width, a first
beam injection time, and a first frequency adjustment factor to provide a
first output dose rate and first energy of a first set of electrons. The
control unit further may be configured to determine a percent
transmission of a first set of x-rays through the container based on an
output of the detector, the first set of x-rays being generated by the
first set of electrons. The control unit also may be configured such that
if the percent transmission is above a predetermined threshold, the
control unit sends a second intensity/energy adjustment command to cause
the intensity controller to determine a second pulse width, a second beam
injection time, and a second frequency adjustment factor to provide a
second output dose rate and second energy of a second set of electrons.
The intensity controller also may be configured to select the second
output dose rate of the second set of electrons such that a dose of a
second set of x-rays generated by the second set of electrons is less
than the dose of the first set of x-rays.

[0016] In alternative embodiments in which the traveling wave linear
accelerator is configured for radiotherapy applications, the traveling
wave linear accelerator may further include an x-ray target configured to
generate x-rays responsive to irradiation with electrons from the
traveling wave linear accelerator, the x-rays being configured to
irradiate a tumor volume; and a robotic arm on which the x-ray target and
the linear accelerator are mounted and configured to adjust an angle at
which the x-rays irradiate the tumor volume.

[0017] The traveling wave linear accelerator may also include a control
unit operatively associated with the robotic arm and with the intensity
controller. The control unit may be configured to send a first
intensity/energy adjustment command to cause the intensity controller to
determine a first pulse width, a first beam injectin time, and a first
frequency adjustment factor to provide a first output dose rate and a
first energy of a first set of electrons, as well as a first position
command to the robotic arm to cause the robotic arm to adjust the angle
to irradiate a first portion of the tumor volume with x-rays generated by
the first set of electrons. The control unit further may be configured to
send a second intensity/energy adjustment command to cause the intensity
controller to determine a second pulse width, a second beam injection
time, and a second frequency adjustment factor to provide a second output
dose rate and a second energy of a second set of electrons; as well as to
send a second position command to the robotic arm to cause the robotic
arm to adjust the angle to irradiate a second portion of the tumor volume
with x-rays generated by the second set of electrons.

[0018] The second energy may be higher than the first energy. The second
tumor volume may be deeper than the first tissue volume. The first tissue
volume and the second tissue volume may receive about the same doses of
x-rays as one another.

[0019] Under another aspect of the present invention, a method is provided
for generating a plurality of dose rates and energies of electrons using
a traveling wave linear accelerator that includes an electron gun
modulator configured to adjust a beam current of electrons from an
electron gun and a frequency controller configured to adjust a frequency
of a signal to be generated. The method may include receiving at an
intensity controller a plurality of intensity/energy adjustment commands
and respectively determining a pulse width, a beam injection time, and a
frequency adjustment factor based on each intensity/energy adjustment
command to provide a respective dose rate and energy of electrons. For
each intensity/energy adjustment command, the method may also include
adjusting the pulse width and beam injection time of electrons from the
electron gun at the electron gun modulator using the determined pulse
width and the determined beam injection time; determining the frequency
of the signal to be generated at the frequency controller using the
frequency adjustment factor; and generating electrons having the
respective output dose rate and energy using the traveling wave linear
accelerator.

[0020] In embodiments where the traveling wave linear accelerator is used
for cargo scanning applications, the method may include sending a first
intensity/energy adjustment command to cause the intensity controller to
determine a first pulse width, a first beam injection time, and a first
frequency adjustment factor to provide a first output dose rate and first
energy of a first set of electrons; generating x-rays with the first set
of electrons; and irradiating a cargo container with the x-rays. The
method also may include determining a percent transmission of a first set
of x-rays through the container based on an output of the detector. If
the percent transmission is below a predetermined threshold, the method
may include sending a second intensity/energy adjustment command to cause
the intensity controller to determine a second pulse width, a second beam
injection time, and a second frequency adjustment factor to provide a
second output dose rate and second energy of a second set of electrons.

[0021] In embodiments where the traveling wave linear accelerator is used
for radiotherapy applications, the method may include sending a first
intensity/energy adjustment command to cause the intensity controller to
determine a first pulse width, a first beam injection time, and a first
frequency adjustment factor to provide a first output dose rate and a
first energy of a first set of electrons; and sending a first position
command to a robotic arm on which the linear accelerator and an x-ray
target are mounted to cause the robotic arm to adjust an angle to
irradiate a first portion of a tumor volume with x-rays generated by the
first set of electrons. The method also may include sending a second
intensity/energy adjustment command to cause the intensity controller to
determine a second pulse width, a second beam injection time, and a
second frequency adjustment factor to provide a second output dose rate
and a second energy of a second set of electrons; and sending a second
position command to the robotic arm to cause the robotic arm to adjust
the angle to irradiate a second portion of the tumor volume with x-rays
generated by the second set of electrons.

5. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention is illustrated by way of example, and not by
way of limitation, in the figures of the accompanying drawings.

[0033]FIG. 9 shows a flow chart of an operation of a TW LINAC comprising
a frequency controller.

[0034]FIG. 10 shows a block diagram of an example computer structure for
use in the operation of a TW LINAC comprising a frequency controller.

[0035]FIG. 11 illustrates a block diagram of a system for scanning cargo
with a TW LINAC.

[0036] FIG. 12 illustrates a flow chart of an operation of the system of
FIG. 11.

[0037]FIG. 13 illustrates a block diagram of a system for performing
radiometry with a TW LINAC.

[0038]FIG. 14 illustrates a flow cart of an operation of the system of
FIG. 13.

6. DETAILED DESCRIPTION

[0039] The present disclosure relates to systems and methods for use in
generating x-rays with modulated pulse-to-pulse dosage, i.e., intensity,
using a traveling wave linear accelerator (TW LINAC), particularly for
use in cargo scanning and radiotherapy applications.

[0040] In an exemplary TW LINAC, electrons injected into an accelerator
structure of the TW LINAC by an electron gun are accelerated and focused
along the accelerator structure using the electric and magnetic field
components of an electromagnetic wave that is coupled into the
accelerator structure. The electromagnetic wave may be coupled into the
accelerator structure from an electromagnetic wave source, such as a
klystron or magnetron. As the electrons traverse the accelerator
structure, they are focused and accelerated by forces exerted on the
electrons by the electric and magnetic field components of the
electromagnetic wave to produce a high-energy electron beam. The electron
beam from accelerator structure may be directed at an x-ray emitting
target to generate x-rays.

[0041] Provided herein are systems and methods for operating a TW LINAC to
generate energy stable electron beams at two or more different
intensities by varying the number of electrons injected into the
accelerator structure during each pulse by, for example, varying the
width of the beam pulse, i.e., pulse width. As discussed further below,
in certain embodiments, concomitant with the electron pulse width
adjustment, adjustments of the injection timing of electron beams on a
pulse-to-pulse basis can advantageously generate electron beams having
substantially the same energy from pulse-to-pulse with varied intensities
in a single energy operation. In a single energy operation,
"pulse-to-pulse" means from one pulse to a subsequent pulse.

[0043] Also provided herein are systems and methods for operating a TW
LINAC to generate energy stable electron beams at two or more different
energies, i.e., an interleaving operation, and at many different
intensities by varying the number of electrons injected into the
accelerator structure during each pulse by, for example, varying the
pulse width. As discussed further below, in certain embodiments,
concomitant with the pulse width adjustment, adjustments of the injection
timing of electron beams on a pulse-to-pulse basis can advantageously
generate electron beams having substantially the same energy from
pulse-to-pulse with varied intensities in a step intensity operation. In
an interleaving energy operation, "pulse-to-pulse" means from one pulse
to the next subsequent pulse having substantially the same energy.

[0044] The electromagnetic wave source in the TW LINAC may be any suitable
radio frequency (RF) source. Non-limiting examples of the electromagnetic
wave source include a klystron, as illustrated in FIG. 1A, and a
magnetron, as illustrated in FIG. 1B. Since a magnetron is an oscillator,
it can be less agile with respect to frequency tuning or power level of
operation than a klystron (an amplifier for which both frequency and
output power can be tuned using a low power external driver). That is, it
can be more difficult to modify the frequency or power level of a
magnetron than a klystron. In certain embodiments, the system and method
need not use the magnetron to vary the frequency or power level of an
electromagnetic wave. The system and method may advantageously facilitate
different energy outputs of the TW LINAC substantially without
modification to the frequency or power level of the magnetron.

[0045] For accelerators that are configured to generate multiple different
energies, the accelerator should be separately tuned at each of the
energy levels to provide maximum efficiency at the highest energy level,
and to maximize stability at each energy level. The following sections
describe a traveling wave linear accelerator (TW LINAC) that can be tuned
at multiple different energy levels to provide a highly stable, highly
efficient x-ray beam. An electromagnetic wave is also referred to herein
as a carrier wave. The electromagnetic waves (i.e., carrier waves)
accelerate electron bunches within an accelerator structure to generate
an x-ray beam. Changing the beam injection timing enables the electron
bunches to be centered on the desired energy even when the pulse width
applied to the electron gun is varied to times relatively short as
compared with the filling time of the structure. Optimizing the frequency
with the frequency controller for the operating energy or energies can
reduce susceptibility of the TW LINAC to jitter of the amplitude and
frequency of the RF electromagnetic waves, jitter of the electron gun at
high voltage and temperature fluctuations of the accelerator structure,
and can maximize efficiency at each energy level.

[0048] In accordance with the principles of the present invention,
intensity controller 13 may be configured to receive a command to adjust
the intensity of the electron beams output from the TW LINAC thereby
adjusting the intensity of x-rays generated by directing the electron
beams at an x-ray emitting target. In one embodiment, the command may be
from a user adjusting a user input device such as a knob, button, switch,
keypad or the like. The intensity controller 13 may be a PLC and/or PC
external to the multi-energy TW LINAC as illustrated. The intensity
controller 13 may be configured to communicate with the PLC or PC
controller. In another embodiment, the intensity controller 13 may be
integrated into the PLC or PC controller of the multi-energy TW LINAC.

[0049] The intensity controller 13 may be further configured to determine
the pulse width and beam injection timing. In one embodiment, the
intensity controller 13 may store predetermined pulse widths and beam
injection timings. Upon receipt of an adjusted intensity command, the
intensity controller 13 may determine a suitable pulse width and beam
injection timing using, for example, a lookup table and/or suitable
computer software for interpolation. The determined pulse width and beam
injection timing may be transmitted by the signal backplane to the gun
modulator 9 such that the gun modulator 9 can change the pulse width and
beam injection timing on a pulse-to-pulse basis.

[0050] The pulse width may scale roughly with the desired intensity for
each energy. Pulse widths may be determined experimentally for each
desired intensity and stored in the lookup table. Pulse widths may also
be interpolated based on the change in intensity using suitable computer
software known to one of ordinary skill in the art.

[0051] The beam injection timing may be determined such that the transient
energy of the pulse is centered around the steady state energy.
Advantageously, electron beams generated with varied intensities using
varied pulse width at such beam injection timing may be stable, even in
the transient beam loading regime. Transient refers to the duration near
or less than about two times the filling time of the TW LINAC. In an
example, the filling time may be about 220 nanoseconds in an X-band TW
LINAC. When beam pulses are shorter than the TW LINAC filling time, the
spectrum quality of the beam may be poor. However, adjusting the beam
injection timing such that the transient energy of the pulse is centered
around the steady state energy may provide high quality electron beams
with substantially the same energy from pulse-to-pulse. Such beam
injection timing may be calculated for a beam with the length of one
filling time or longer as follows:

where tinj is the beam injection timing, i0 is current in a
constant gradient traveling wave accelerator, τ is attenuation
factor, E0 is accelerating gradient, r is shunt impedance, Q is
quality factor, and ω is RF angular frequency. For a beam length
less than the filling time, the beam injection time may be adjusted based
on the best optimized energy spectrum, which may be centered with the
steady state energy gain.

[0052]FIG. 1c is a plot showing the energy gain of an electron beam with
current io in a constant gradient TW LINAC. In the Figure, tb
is the total length of the beam in time, tF is filling time, ti
is the start time of the beam, and l is length. As seen in the Figure,
the exemplary beam injection timing may be determined such that the
transient energy of the pulse is centered around the steady state energy.

[0053] Beam injection timings may be determined experimentally for each
desired intensity and stored in the lookup table. Beam injection timings
may also be interpolated based on the change in intensity using suitable
computer software known to one of ordinary skill in the art.

[0054] In embodiments where the TW LINAC includes a magnetron (discussed
below with reference to FIG. 1B), the beam injection time may be based on
the rise time of the RF pulse set on the magnetron. In other embodiments
where the TW LINAC includes a klystron, the rise time of the RF pulse may
be adjusted such that the fields in the LINAC are very similar to steady
state beam-loaded fields when the beam is first injected. Appropriate
choice of the RF rise time combined with the appropriate beam injection
timing of the gun pulse, determined as described herein, can reduce or
even eliminate the energy transient associated with beam loading.
Advantageously, selecting proper RF rise time and beam injection time
allows x-ray intensity to be changed without changing x-ray energy. The
optimum rise time of an RF pulse depends on the beam current which
changes for different energies, but not for different intensities when
the intensity is adjusting by changing pulse length. For example, a rise
time may be determined for an RF source by adjusting the rise time and
observing the energy spectrum of the resulting beam on an energy analyzer
instrument, such as a bending magnet energy analyzer.

[0055] The beam current may be held constant for single energy LINAC
operations. For interleaving operations, the beam current may be held
constant from pulse-to-pulse and may vary for different output energies.
Beam currents may be calculated or determined experimentally for each
desired energy and stored in the lookup table.

[0056] The frequency does not change for a change in intensity, but
changes for a change in energy.

[0057] The intensity controller 13 may include a computer readable medium
including instructions that, when executed by a processor, cause the
processor to determine and transmit the pulse width and the beam
injection time as discussed above. Non-limiting examples of a computer
readable medium include a floppy disk, a hard disk, a memory, RAM, ROM, a
compact disk, a digital video disk, and the like. The computer readable
medium may be further configured to adjust the pulse width and beam
injection time of electrons from an electron gun 14 using the determined
pulse width and the determined beam injection time. The TW LINAC may then
generate an output dose rate of electrons.

[0058] In some embodiments, the intensity controller 13 may include and
may execute a programmed routine configured to receive an intensity
adjustment command and to determine the pulse width and the beam
injection time as discussed above.

[0059] Through the control interface, a user can adjust settings, control
operation, etc. of the TW LINAC. The control interface communicates with
a programmable logic controller (PLC) and/or a personal computer (PC)
that is connected to a signal backplane. The PLC and/or PC may include
the computer readable medium and/or the processor and may execute the
programmed routine discussed above. The signal backplane provides control
signals to multiple different components of the TW LINAC based on
instructions received from the PLC, PC and/or control interface.

[0060] A frequency controller 1 receives phase tracking and tuning control
information from the signal backplane. The frequency controller 1 can be
configured to operate at a single frequency setting or to alternate
between two or more different frequency settings. For example, the
frequency controller 1 can be configured to alternate between a frequency
of 9290 MHz and a frequency of 9291 MHz, 400 times per second.
Alternatively, the frequency controller 1 may be configured to alternate
between more than two different frequencies. In an example, based on the
comparison of the measured phase shift of the electromagnetic wave
through the TW LINAC on the previous pulse of the same energy with the
set point for energy of the next pulse, the frequency controller 1
adjusts settings of an oscillator 2. By modifying the frequency of the RF
signal generated by the oscillator 2, the frequency controller 1 can
change the frequency of electromagnetic waves (carrier waves) produced by
a klystron 6 on a pulse by pulse basis. Frequency shifts on the order of
one or a few parts in 10,000 can be achieved.

[0061] The frequency controller 1 may be a phase detection frequency
controller, and can use phase vs. frequency response to establish a
correct frequency setting. The frequency controller 1, by monitoring and
correcting the phase shift from the input to the output of the
accelerator, can correct for medium and slow drifts in either the RF
frequency or the temperature of the accelerator structure 8. The
frequency controller 1 can operate as an automatic frequency control
(AFC) system. In an example, the frequency controller 1 can be a
multi-frequency controller, and can operate at a set point for each of
several different frequencies, with each frequency being associated with
each different energy. The frequency controller, including the AFC, is
discussed further in Section 6.3 below.

[0062] The oscillator 2 generates an RF signal having a frequency that is
provided by the frequency controller 1. The oscillator 2 is a stable low
level tunable RF source that can shift in frequency rapidly (e.g.,
between pulses generated by the klystron modulator 4). The oscillator 2
can generate an RF signal at the milliwatt level. The RF signal is
amplified by an amplifier 3 (e.g., a 40 Watt amplifier), and supplied to
a klystron 6.

[0063] The amplifier 3 can be a solid state amplifier or a traveling wave
tube (TWT) amplifier, and can amplify the received RF signal to a level
required for input to the klystron 6. The amplifier 3 may be configured
to receive RF pulse rise time information from the signal backplane as
determined by the intensity controller 13. In an example, the amplifier 3
can be configured to change the output power level, on a pulse to pulse
basis, to the level appropriate for the energy of an upcoming LINAC
pulse. Alternatively, the klystron modulator 4 could deliver different
high voltage pulses to the klystron 6 for each beam energy required.

[0064] A klystron modulator 4 receives heater and high voltage (HV) level
control, trigger pulse and delay control, startup and reset, and sensing
and interlock signals from the signal backplane. The klystron modulator 4
is a capable of generating high peak power pulses to a pulse transformer.
The effective output power of the klystron modulator 4 is the power of
the flat-top portion of the high voltage output pulse. The klystron
modulator 4 can be configured to generate a new pulse at each frequency
change in the frequency controller 1. For example, a first pulse may be
generated when the frequency controller 1 causes the oscillator 2 to
generate an RF signal having a first frequency, a second pulse may be
generated when the frequency controller 1 causes the oscillator 2 to
generate an RF signal having a second frequency, a third pulse may be
generated when the frequency controller 1 causes the oscillator 2 to
generate an RF signal having the first frequency, and so on.

[0065] The klystron modulator 4 drives energy into a pulse transformer 5
in the form of repeated high energy approximately square wave pulses. The
pulse transformer 5 increases the received pulses into higher voltage
pulses with a medium to high step-up ratio. The transformed pulses are
applied to the klystron 6 for the generation of high energy microwave
pulses. The rise time of the output pulse of the klystron modulator 4 is
dominated by the rise time of the pulse transformer 5, and therefore the
pulse transformer 5 is configured to have a fast rise time to approximate
square waves.

[0066] The klystron 6 is a linear-beam vacuum tube that generates high
power electromagnetic waves (carrier waves) based on the received
modulator pulses and the received oscillator radio frequency (RF) signal.
The klystron 6 provides the driving force that powers the linear
accelerator. The klystron 6 coherently amplifies the input RF signal to
output high power electromagnetic waves that have precisely controlled
amplitude, frequency and input to output phase in the TW LINAC
accelerator structure. The klystron 6 operates under pulsed conditions,
which enables the klystron 6 to function using a smaller power source and
require less cooling as compared to a continuous power device. The
klystron 6 typically has a frequency band width on the order of one
percent or more.

[0067] The klystron 6 is a high-gain amplifier, therefore, the output RF
signal generated by the klystron 6 has the same frequency as the low
power RF signal input to the klystron 6. Thus, changing the frequency of
the high power RF electromagnetic wave used to drive the LINAC can be
achieved simply by changing the frequency of the low power RF signal used
to drive the klystron 6. This can be easily performed between pulses with
low power solid state electronics. Similarly, the output power of the
electromagnetic wave from the klystron can be changed from pulse to pulse
by just changing the power out of the amplifier 3. Moreover, the rise
time of the klystron output pulse can be determined by the rise time of
the low power RF pulse used to drive the klystron. This can reduce or
even eliminate the beam loading transient experienced in the LINAC.

[0068] A waveguide 7 couples the klystron 6 to an input of an accelerator
structure 8 of the TW LINAC. The waveguide 7 includes a waveguide coupler
and a vacuum window. The waveguide 7 carries high powered electromagnetic
waves (carrier waves) generated by the klystron 6 to the accelerator
structure 8. The waveguide coupler of waveguide 7 can sample a portion of
the electromagnetic wave power to the input of the LINAC. A waveguide 12
that includes a waveguide coupler and a vacuum window couples the output
of the accelerator structure 8 to the RF load. The waveguide coupler of
waveguide 12 can sample a portion of the electromagnetic wave power from
the output of the LINAC. A phase comparator of frequency controller 1 can
be used to compare a signal from the waveguide coupler of waveguide 7 to
a signal from the waveguide coupler of waveguide 12 to determine the
phase shift of the electromagnetic wave through accelerator structure 8.
The frequency controller 1 uses the phase shift of the electromagnetic
wave to determine the frequency correction to be applied at the klystron,
if any. Waveguide 7 or waveguide 12 can be a rectangular or circular
metallic pipe that is configured to optimally guide waves in the
frequencies that are used to accelerate electrons within the LINAC
without significant loss in intensity. The metallic pipe can be a low-Z,
high conductivity, material such as copper. To provide the highest field
gradient possible with near maximum input power, the waveguide coupler
can be filled with SF6 gas. Alternatively, the waveguide can be
evacuated.

[0069] The vacuum window permits the high power electromagnetic waves to
enter the accelerator structure 8 while separating the evacuated interior
of the accelerator structure 8 from its gas filled or evacuated exterior.

[0070] A gun modulator 9 controls an electron gun 14 that fires electrons
into the accelerator structure 8. The gun modulator 9 receives grid drive
level and current feedback control signal information from the signal
backplane. The gun modulator 9 further receives gun trigger pulses and
delay control pulse and gun heater voltage and HV level control from the
signal backplane. The gun modulator 9 controls the electron gun 14 by
instructing it when and how to fire (e.g., including repetition rate and
grid drive level to use). The gun modulator 9 can cause the electron gun
14 to fire the electrons at a pulse repetition rate that corresponds to
the pulse repetition rate of the high power electromagnetic waves
(carrier waves) supplied by the klystron 6. The gun modulator 9 is
operatively associated with the intensity controller 13. Preferably, the
gun modulator 9 causes the electron gun 14 to fire the electrons at a
pulse width(s) and beam injection time(s) determined by the intensity
controller 13.

[0071] An exemplary electron gun includes an anode, a grid, a cathode and
a filament. The filament is heated to cause the cathode to release
electrons, which are accelerated away from the cathode and towards the
anode at high speed. The anode can focus the stream of emitted electrons
into a beam of a controlled diameter. The grid can be positioned between
the anode and the cathode.

[0072] The electron gun 14 is followed by a buncher that is located after
the electron gun 14 and is typically integral with the accelerating
structure. In one embodiment, the buncher is composed of the first few
cells of the accelerating structure. The buncher packs the electrons
fired by the electron gun 14 into bunches and produces an initial
acceleration. Bunching is achieved because the electrons receive more
energy from the electromagnetic wave (more acceleration) depending on how
near they are to the crest of the electromagnetic wave. Therefore,
electrons riding higher on the electromagnetic wave catch up to slower
electrons that are riding lower on the electromagnetic wave. The buncher
applies the high power electromagnetic waves provided by the klystron 6
to the electron bunch to achieve electron bunching and the initial
acceleration.

[0073] High power electromagnetic waves are injected into the accelerator
structure 8 from the klystron 6 via the waveguide 7. Electrons to be
accelerated are injected into the accelerator structure 8 by the electron
gun 14. The electrons enter the accelerator structure 8 and are typically
bunched in the first few cells of the accelerator structure 8 (which may
comprise the buncher). The accelerator structure 8 is a vacuum tube that
includes a sequence of tuned cavities separated by irises. The tuned
cavities of the accelerator structure 8 are bounded by conducting
materials such as copper to keep the RF energy of the high power
electromagnetic waves from radiating away from the accelerator structure
8.

[0074] The tuned cavities are configured to manage the distribution of
electromagnetic fields within the accelerator structure 8 and
distribution of the electrons within the electron beam. The high power
electromagnetic waves travel at approximately the same speed as the
bunched electrons so that the electrons experience an accelerating
electric field continuously. In the first portion of the TW LINAC, each
successive cavity is longer than its predecessor to account for the
increasing particle speed. Typically, after the first dozen or so cells
the electrons reach about 98% of the velocity of light and the rest of
the cells are all the same length. The basic design criterion is that the
phase velocity of the electromagnetic waves matches the particle velocity
at the locations of the accelerator structure 8 where acceleration
occurs.

[0075] Once the electron beam has been accelerated by the accelerator
structure 8, it can be directed at a target, such as a tungsten or copper
target, that is located at the end of the accelerator structure 8. The
bombardment of the target by the electron beam generates a beam of x-rays
(discussed in Section 6.4 below). The electrons can be accelerated to
different energies before they strike a target. In an interleaving
operation, the electrons can be alternately accelerated to two different
output energies, e.g., to 6 mega electron volts (MeV)1 and to 9 MeV.
Alternately, the electrons can be accelerated to different energies.
1 One electron volt equals 1.602×10-19 joule. Therefore,
6 MeV=9.612×10-13 joule per electron.

[0076] To achieve a light weight and compact size, the TW LINAC may
operate in the X-band (e.g., at an RF frequency between 8 GHz and 12.4
GHz). The high operating frequency, relative to a conventional S-band
LINAC, reduces the length of the accelerator structure 8 by approximately
a factor of three, for a given number of accelerating cavities, with a
concomitant reduction in mass and weight. As a result, all of the
essential components of the TW LINAC may be packaged in a relatively
compact assembly. Alternatively, the TW LINAC may operate in the S-band.
Such a TW LINAC requires a larger assembly, but can provide a higher
energy x-ray beam (e.g., up to about 18 MeV) with commercially available
high power electromagnetic wave sources.

[0077] A focusing system 10 controls powerful electromagnets that surround
the accelerator structure 8. The focusing system 10 receives a current
level control from the signal backplane, and controls a current level of
focusing coils to focus an electron beam that travels through the
accelerator structure 8. The focusing system 10 is designed to focus the
beam to concentrate the electrons to a specified diameter beam that is
able to strike a small area of the target. The beam can be focused and
aligned by controlling the current that is supplied to the electromagnet.
In an example, the focusing current is not changed between pulses, and
the current is maintained at a value which allows the electromagnet to
substantially focus the beam for each of the different energies of
operation.

[0078] A sulfur hexafluoride (SF6) controller controls an amount
(e.g., at a specified pressure) of SF6 gas that can be pumped into
the waveguide. The SF6 controller receives pressure control
information from the backplane and uses the received information to
control the pressure of SF6 gas that is supplied to the waveguide.
SF6 gas is a strong electronegative molecule, giving it an affinity
for free electrons. Therefore, the SF6 gas is used as a dielectric
gas and insulating material, and can be provided to waveguide 7 and
waveguide 12 to quench arcs that might otherwise occur. The SF6 gas
increases the amount of peak power that can be transmitted through the
waveguide 7, and can increase the voltage rating of the TW LINAC.

[0079] A vacuum system (e.g., an ion pump vacuum system) can be used to
maintain a vacuum in both the klystron 6 and the accelerator structure 8.
A vacuum system also can be used to generate a vacuum in portions of the
waveguide 7. In air, intense electric and magnetic fields cause arcing,
which destroys the microwaves, and which can damage the klystron,
waveguide or accelerator structure. Additionally, within the accelerator
structure 8, any beams that collide with air molecules are knocked out of
the beam bunch and lost. Evacuating the chambers prevents or minimizes
such occurrences.

[0080] The vacuum system may report current vacuum levels (pressure) to
the signal backplane. If pressure of the klystron 6 or accelerator
structure 8 exceed a pressure threshold, the vacuum system may transmit a
command to the signal backplane to turn off the klystron 6 until an
acceptable vacuum level is reached.

[0081] Many components of the TW LINAC can generate heat. Heat can be
generated, for example, due to the electromagnetic wave power loss on the
inner walls of the accelerator, by the electron bombardment of the target
at the end of the accelerator structure 8, and by the klystron 6. Since
an increase in temperature causes metal to expand, temperature changes
affect the size and shape of cavities within the accelerator structure,
the klystron, the waveguide, etc. This can cause the frequency at which
the wave is synchronous with the beam to change with the temperature. The
proper operation of the accelerator requires careful maintenance of the
cavity synchronous frequency to the passage of beam bunches. Therefore, a
cooling system 11 is used to maintain a constant temperature and minimize
shifts in the synchronous frequency.

[0082] The cooling system 11 circulates water or other coolant to regions
that need to be cooled, such as the klystron 6 and the accelerator
structure 8. Through the signal backplane, the cooling system 11 receives
water flow rate and temperature control information. The cooling system
11 can be used to monitor the temperature of the klystron 6 and the
accelerator structure 8, and can be configured to maintain a constant
temperature in these components. However, the temperature of the metal of
the accelerator structure and the klystron may rise as much as 10 degrees
when the LINAC is operated at a high repetition rate, which can
contribute to the drift in the electromagnetic wave. The frequency
controller can be used to compensate for the effect of the drift.

[0083]FIG. 1B illustrates a block diagram of an exemplary multi-energy
traveling wave linear accelerator including a magnetron, in accordance
with one embodiment of the present invention. Many components of FIG. 1B
are described above with respect to FIG. 1A. The exemplary multi-energy
TW LINAC of FIG. 1B further includes tuner 15, magnetron 16, and
magnetron modulator 17 and does not include oscillator 2, amplifier 3,
klystron modulator 4, and klystron 6 illustrated in FIG. 1A. Magnetron
modulator 17 may operate in substantially the same manner as klystron
modulator 4 described above with respect to FIG. 1B. Magnetron modulator
17, tuner 15, and magnetron 16 may include the features hereinafter
described, but otherwise may be conventional.

[0084] The magnetron 16 may function as a high-power oscillator, to
generate electromagnetic waves (usually microwave) pulses of several
microseconds duration and with a repetition rate of several hundred
pulses per second. The frequency of the electromagnetic waves within each
pulse can be typically about 3,000 MHz (S-band) or about 9,000 MHz
(X-band). For very high peak beam currents or high average currents, 800
to 1500 MHz (L-band) pulses can be used. The magnetron can be any
magnetron deemed suitable by one of skill. For example, the CTL X-band
pulsed magnetron, model number PM-1100X (L3 Communications, Applied
Technologies, Watsonville, Calif.) can be used.

[0085] Typically, the magnetron has a cylindrical construction, having a
centrally disposed cathode and an outer anode, with resonant cavities
machined out of a solid piece of copper. The space between the centrally
disposed cathode and the outer anode can be evacuated. The cathode can be
heated by an inner filament; the electrons are generated by thermionic
emission. A static magnetic field can be applied perpendicular to the
plane of the cross-section of the cavities (for example, perpendicular to
a pulsed DC electric field), and a pulsed DC electric field applied
between the cathode and the anode. The electrons emitted from the cathode
can be accelerated toward the anode by the action of the pulsed DC
electric field and under the influence of the magnetic field. Thus, the
electrons can be moved in a complex spiraling motion towards the resonant
cavities, causing them to radiate electromagnetic radiation at a
frequency in the microwave region of the electromagnetic spectrum. The
generated microwave pulses can be coupled into to an accelerator
structure via a transfer waveguide.

[0086] Magnetrons can operate at 1 or 2 MW peak power output to power
low-energy LINACs (6 MV or less). Magnetrons can be relatively
inexpensive and can be made compact, which can be an advantage for many
applications. Continuous-wave magnetron devices can have an output power
as high as about 100 kW at 1 GHz with efficiencies of about 75-85
percent, while pulsed devices can operate at about 60-77 percent
efficiency. Magnetrons can be used in single-section low energy linear
accelerators that may not be sensitive to phase. Feedback systems can be
interfaced with the magnetron to stabilize the frequency and power of the
electromagnetic wave output.

[0087] The frequency controller 1 can be used to maintain the phase shift
through the LINAC at the same set point for the different energies of
operation of the LINAC. Specifically, the frequency controller 1 can
transmit a signal to the tuner 15 to tune the magnetron in order to
maintain the phase shift of the electromagnetic wave at the set point.
For example, if the measured phase shift of the first electromagnetic
wave (generated at a first frequency) is not at the set point, the
frequency controller 1 can transmit a signal to the tuner 15 to tune the
magnetron to generate a second electromagnetic wave at a modified
frequency (i.e., to a second frequency that is not equal to the first
frequency) to cause the phase shift of the second electromagnetic wave to
be closer to the set point. The first frequency and the second frequency
are different if they differ by greater than about 0.001% in magnitude,
or more. If the measured phase shift of the first electromagnetic wave
(generated at a first frequency) is at the set point, the frequency
controller 1 can transmit a signal to the tuner 15 so that the magnetron
to generate the second electromagnetic wave at substantially the same
frequency as the first electromagnetic wave. For example, the first
frequency and the second frequency can be substantially the same
frequency if they differ by less than about 0.001%. That is, a
measurement of the phase difference between P1 and P2 can cause the
magnetron to be tuned to alter its operating frequency, if necessary, and
thereby maintain a specific phase shift of the electromagnetic waves
through the accelerator structure.

[0088] Thus, the signal from the frequency controller 1 to the magnetron
can ultimately result in maintaining the phase shift of the
electromagnetic waves through the accelerator structure at a set point,
based on the magnitude of the phase shift detected by the frequency
controller. In a non-limiting example, the frequency controller can be an
automatic frequency controller (AFC). The frequency controller may
include a controller and a phase comparator as an integral unit. However,
in other embodiments, the frequency controller 1 can comprise the
controller and phase comparator as separate units.

[0089] The frequency of the electromagnetic wave generated by the
magnetron can be tuned mechanically. For example, a tuning pin or a
tuning slug positioned in communication with the body of the magnetron
can be moved in or out of the body of the magnetron to tune its operating
frequency. Tuner 15 can include a motor drive that moves the tuning pin
or tuning slug to tune the magnetron mechanically. In an embodiment where
the magnetron is operated to generate electromagnetic waves at
substantially a single frequency (or at values of frequency (f) within a
range (δf) around the single frequency), the mechanical tuning can
be used to maintain the stability of the performance of the magnetron.
For example, δf can be a difference on the order of about one or a
few parts in 10,000 of a frequency in kHz. In some embodiments, δf
can be a difference on the order of about 0.01 MHz or more. As described
in greater detail below, the frequency controller can be used to maintain
the stability of the output energy and electron dose stability.

[0090] When the TW LINAC is operated at two or more different energies,
the magnetron can be tuned to operate at a range of values (δf)
around a single frequency (f) that provides for a maximized output of the
LINAC at all of the different energies of operation. For example, in an
embodiment where the LINAC is operated at 6 MeV and 9 MeV, the magnetron
can be operated to generate electromagnetic waves at values within a
range (δf) around a single frequency (f) such that the electron
bunches are accelerated on average slightly ahead of the peak of the
electromagnetic wave during the 9 MeV operation and are accelerated on
average slightly behind the peak of the electromagnetic wave during the 6
MeV. In a TW LINAC, the energy can be changed by a significant amount,
for example from 9 MeV to 6 MeV, by changing the gun current while only
having a small effect on the RF fields in the buncher. The percentage
change of the fields in the buncher is much smaller than the 33%
reduction in the energy because the beam induced fields which lower the
energy are zero at the input end and rise roughly linearly with distance
through the LINAC. This means that changing the beam energy in a TW LINAC
by changing the beam current should have only a small effect on the phase
of the bunch relative to the RF traveling wave in the LINAC.

[0091] The single frequency of operation of the magnetron can be
determined by first finding an intermediate electron gun current between
those used for the two different energies of operation, for which
adjusting the frequency of the magnetron to optimize the x-ray yield of
the LINAC provides acceptable energy spectrum and stability for both the
highest energy operation and the lowest energy. The intermediate electron
gun current can be, but is not limited to, an average or median of the
highest electron gun current and the lowest electron gun current for a
two-energy operation or for operation at three or more different
energies. The single frequency of operation of the magnetron, and the
range of values (δf) around the single frequency, can be determined
as the frequency (and δf) that maximizes a x-ray yield of the LINAC
for that intermediate electron gun current. The frequency controller can
facilitate stable operation during rapid switching of a multi-energy
interleaved operation of the TW LINAC. The frequency controller can be
used to correct for the effect of rapid thermalization of the TW LINAC
accelerator structure when the system is stepping from standby to full
power, drifts in the temperature of the accelerator structure cooling
water, or drifts in the frequency of the magnetron.

[0092]FIG. 2 illustrates a cross-section of a target structure 20 coupled
to the accelerator structure 8 (partially shown). The target structure 20
includes a target 22 to perform the principal conversion of electron
energy to x-rays. The target 22 may include a low-Z material such as but
not limited to copper, which can avoid or minimize generation of neutrons
when bombarded by the output electrons. Additionally, the target may be,
for example, an alloy of tungsten and rhenium, where the tungsten is the
principle source of x-rays and the rhenium provides thermal and
electrical conductivity. The target 22 may include one or more target
materials having an atomic number approximately greater than or equal to
70 to provide efficient x-ray generation.

[0093] When electrons from the electron beam enter the target, they give
up energy in the form of heat and x-rays (photons), and lose velocity. In
operation, an accelerated electron beam impinges on the target,
generating Bremsstrahlung and k-shell x-rays (see Section 6.4 below).

[0094] The target 22 may be mounted in a metallic holder 24, which may be
a good thermal and electrical conductor, such as copper. The holder 24
may include an electron collector 26 to collect electrons that are not
stopped within the target 22 and/or that are generated within the target
22. The collector 26 may be a block of electron absorbing material such
as a conductive graphite based compound. In general, the collector 26 may
be made of one or more materials with an atomic number approximately less
than or equal to 6 to provide both electron absorption and transparency
to x-rays generated by the target 22. The collector 26 may be
electrically isolated from a holder by an insulating layer 28 (e.g., a
layer of anodized aluminum). In an example, the collector 26 is a heavily
anodized aluminum slug.

[0095] A collimator 29 can be attached to the target structure. The
collimator 29 shapes the x-ray beam into an appropriate shape. For
example, if the TW LINAC is being used as an x-ray source for a cargo
inspection system, the collimator 29 may form the beam into a fan shape.
The x-ray beam may then penetrate a target (e.g., a cargo container), and
a detector at an opposite end of the target may receive x-rays that have
not been absorbed or scattered. The received x-rays may be used to
determine properties of the target (e.g., contents of a cargo container).

[0096] An x-ray intensity monitor 31 can be used to monitor the yield of
the x-ray during operation (see FIG. 2). A non-limiting example of an
x-ray intensity monitor 31 is an ion chamber. The x-ray intensity monitor
can be positioned at or near the x-ray source, for example, facing the
target. In one embodiment, based on measurements from the x-ray intensity
monitor 31 from one pulse of the LINAC to another, the frequency
controller can transmit a signal to the one or more oscillators to cause
the electromagnetic wave source to generate an electromagnetic wave at a
frequency to maximize the yield of x-ray at an energy.

[0097] The frequency controller 1 can be interfaced with the x-ray
intensity monitor 31. The frequency controller 1 can be used to monitor
the measurements from the x-ray intensity monitor (which provide an
indication of the x-ray yield) and use that information to provide a
signal to the oscillator. The oscillator can tune the electromagnetic
wave source to generate an electromagnetic wave at a frequency based on
the signal from the frequency controller. In an embodiment, the frequency
controller may be configured to compare a measurement from the x-ray
intensity monitor that indicates the yield of the first beam of x-rays
emitted in a desired range of x-ray energies to a measurement from the
x-ray intensity monitor that indicates the yield of the second beam of
x-rays at that range of x-ray energies. The second beam of x-rays can be
generated using a set of electrons that is accelerated in the accelerator
structure by an electromagnetic wave that has about the same amplitude as
that used in the generation of the first beam of x-rays. For example, the
electromagnetic waves can have about the same magnitude if they differ by
less than about 0.1%, less than about 1%, less that about 2%, or more.
The frequency of the electromagnetic wave delivered to the LINAC for
generating the second beam of x-rays can differ in magnitude from the
frequency of the electromagnetic wave delivered to the LINAC for
generating the first beam of x-rays by a small amount (δf). For
example, δf may be a difference on the order of about one or a few
parts in 10,000 of a frequency in Hz. In some embodiments, δf can
be a difference on the order of about 0.001 MHz or more, about 0.01 MHz
or more, about 0.03 MHz or more, about 0.05 MHz or more, about 0.08 MHz
or more, about 0.1 MHz or more, or about 0.15 MHz or more. The frequency
controller can transmit a signal to the oscillator so that the oscillator
causes the electromagnetic wave source to generate a subsequent
electromagnetic wave at a frequency to maximize the yield of a x-rays in
a subsequent operation of the LINAC.

[0098] The frequency controller can tune the frequency of the
electromagnetic wave by monitoring both (i) the phase shift of the
electromagnetic wave from the input to the output of the accelerator
structure and (ii) the dose from the x-ray intensity monitor.

[0099] In another embodiment, the frequency controller can also be
interfaced with an electron energy spectrum monitor 27 (see FIG. 2). A
non-limiting example of an electron energy spectrum monitor is an
electron absorber followed by an electron current monitor. For example,
an electron current monitor can be configured to measure the current
reaching the electron current collector 26 in the target assembly (see
FIG. 2). The electron energy spectrum monitor can be positioned near the
output of the accelerator structure. The electron energy spectrum monitor
can be used to monitor the electron current of the output of electrons
for a given pulse of the LINAC. Based on the measurements from the
electron energy spectrum monitor, the frequency controller transmits a
signal to the oscillator so that the oscillator tunes the electromagnetic
wave source to the desired frequency. In this embodiment, the frequency
controller can be configured to compare an indication of a first energy
spectrum of a first output of electrons from the output of the
accelerator structure to an indication of a second energy spectrum of a
second output of electrons from the output of the accelerator structure,
and transmit a signal to the oscillator based on the comparison. For
example, the frequency controller can be configured to compare a first
electron current of the first output of electrons from one pulse of the
LINAC to a second electron current of the second output of electrons from
another pulse. The second output of electrons can be generated using an
electromagnetic wave that has about the same amplitude as that used to
generate the first output of electrons. For example, the electromagnetic
waves can have about the same magnitude if they differ by less than about
0.1%, less than about 1%, less that about 2%, or more. The frequency of
the electromagnetic wave delivered to the LINAC for generating the second
output of electrons can differ in magnitude from the frequency of the
electromagnetic wave delivered to the LINAC for generating the first
output of electrons by a small amount (δf). For example, δf
may be a difference on the order of about one or a few parts in 10,000 of
a frequency in Hz. In some embodiments, δf can be a difference on
the order of about 0.001 MHz or more, about 0.01 MHz or more, about 0.03
MHz or more, about 0.05 MHz or more, about 0.08 MHz or more, about 0.1
MHz or more, or about 0.15 MHz or more. Based on the signal from the
frequency controller, the oscillator can cause the electromagnetic wave
source to generate a subsequent electromagnetic wave at a frequency to
stabilize the energy of a subsequent output of electrons.

[0100] In an embodiment, the frequency controller can tune the frequency
of the electromagnetic wave by monitoring both (i) the phase shift of the
electromagnetic wave from the input and the output of the accelerator
structure and (ii) the electron current of the output of electrons.

[0101] In yet another embodiment, the frequency controller can tune the
electromagnetic wave source primarily by monitoring the phase shift of
the electromagnetic wave from the input and the output of the accelerator
structure, and as a secondary measure can monitor the doses of the x-ray
intensity monitor and the electron current of the output of electrons.

[0102] The frequency controller can be configured to tune the frequency of
the electromagnetic wave source, based on the monitoring of the phase,
x-ray yield, and/or energy spectrum of the output electrons from pulses
of the LINAC as described herein, in an iterative process. That is, the
frequency controller can be configured to tune the electromagnetic wave
source in an iterative process so that, with each subsequent pulse of the
LINAC for a given energy of operation, the yield of x-rays is further
improved until it reaches the maximum or is maintained at the maximum, or
the stability of the energy spectrum of the output of electrons is
further increased or maintained.

6.2 Multi-Energy Traveling Wave Linear Accelerator Operation Theory

[0103] In a one energy LINAC, the accelerator structure 8 is configured
such that the electron bunch rides at the crest of the high energy
electromagnetic waves throughout the accelerator structure 8, except in
the first few cells of the accelerator structure 8 that comprise the
buncher. This can be accomplished by ensuring that the electric field of
the electromagnetic waves remains in phase with the electron bunches that
are being accelerated. An electron bunch that rides at the crest of the
electromagnetic wave receives more energy than an electron bunch that
rides off the crest, which increases efficiency of the LINAC. Moreover,
the crest of the electromagnetic wave has a slope of zero. Therefore, if
jitter occurs to cause the electron bunch to move off of the crest of the
wave, the amount of energy imparted to the electron bunch changes only by
a very small amount. Furthermore, the bunch has a finite length. If it
rides at the crest which has zero slope the electron beam will have a
narrower spectrum. For these reasons, it is desirable to have the
electron bunch ride the crest of the electromagnetic waves.

[0104]FIG. 3 illustrates an electron bunch 30 riding an electromagnetic
wave 32 (also referred to as a carrier wave) at the beginning of the
accelerator structure (just after exiting the buncher), at the middle of
the accelerator structure, and at the end of the accelerator structure
(just before striking the target). FIG. 3 illustrates a higher energy
operation of the LINAC, where electron bunch 30 can ride substantially at
the crest of the electromagnetic wave 32 at each region of the
accelerator structure (substantially synchronous).

[0105] In a multi-energy LINAC, the accelerator structure is typically
configured such that at the higher energy operation the electron bunches
30 ride at the crests of the high energy electromagnetic waves 32, as is
shown in FIG. 3. However, to impart less energy on the electron beam for
the lower energy operation, the strength (amplitude) of the
electromagnetic wave can be reduced by reducing the output power of the
klystron 6 (e.g., by reducing the input drive power to the klystron 6 or
by reducing the klystron high voltage pulse). As another exemplary way to
impart less energy on the electron beam for the lower energy operation,
the acceleration imparted by the electromagnetic wave also can be reduced
by increasing the beam current from the electron gun 14 in an effect
referred to as beam loading (described in Section 6.3 below). The lower
strength electromagnetic wave accelerates the electron bunches at a
slower rate than the higher strength electromagnetic waves. Therefore,
when the RF field amplitude is lowered to lower the energy of the x-ray
beam, the electron bunches gain energy less rapidly in the buncher and so
end up behind the crest of the wave at the end of the buncher. This
causes the electron bunches to fall behind the crest of the waves by the
end of the buncher region of the accelerator structure. If the RF
frequency is the same for the low energy level as for the high energy
level, the bunch will stay behind the crest in the accelerator structure,
resulting in a broad, undesirable, energy spectrum.

[0106] When the electron bunch does not travel at the crest of the
electromagnetic wave, the efficiency of the LINAC is reduced, and
therefore greater power is required than would otherwise be necessary to
generate the lower power x-ray beam. More importantly, since the electron
bunch is not at the crest of the wave, any jitter can cause the electron
bunch to move up or down on the electromagnetic sine wave. Thus, the
energy of the x-ray beam will fluctuate in response to phase fluctuations
caused by jitter in the RF frequency, variation in the gun voltage or
current, and amplitude and variation in the accelerator structure
temperature. This changes the amount of energy that is imparted to the
electron bunch, which causes instability and reduces repeatability of the
resultant x-ray beam. However, reducing the beam energy by increasing the
beam current has a significantly different effect than by reducing the
input drive power. When the input power is reduced, the RF fields
throughout the LINAC decrease by the same percentage. When the beam
current is increased in a TW LINAC, the percentage decrease at the input
end of the LINAC is very small and the change increases roughly linearly
with distance along the LINAC. Most of the decrease in energy occurs
after the electrons are relativistic and has very little effect on their
velocities. Consequently, when a two energy operation is achieved by
changing the gun current, it can be quite satisfactory to accelerate both
of the energies beams with the same frequency RF power, which is a
compromise placing the higher energy slightly ahead of the crest and the
lower energy beam slightly behind the crest. In one simulation with two
energies, 6 MeV and 4 MeV, were run with the same RF power and the same
RF frequency. The 4 MeV beam was obtained using a higher gun current
(about 100 mA higher captured beam current). The 6 MeV bunch ended up
about 8° ahead of the crest and the 4 MeV beam was 8°
behind the crest. Each beam lost about 1% energy by not being on the
crest and both had good spectra.

[0107] Three typical sources of jitter include frequency jitter from the
RF source, temperature variation from the accelerator structure and
amplitude jitter from the RF source. All three sources of jitter can
cause the electron bunch to move up or down on the electromagnetic sine
wave. Additionally, amplitude jitter of the RF source also can cause
jitter in the amplitude of the accelerating fields throughout the LINAC.

[0108] A standing wave LINAC has a fixed number of half wavelengths from
one end of the accelerator structure to the other, equal to the number of
resonant accelerating cavities. Therefore, the phase velocity of the
electromagnetic waves cannot be changed in a standing wave LINAC. For the
standing wave LINAC, when the frequency of the electromagnetic wave is
changed, the electromagnetic wave moves off the resonance frequency of
the accelerator structure, and the amplitude of the electromagnetic waves
decreases. However, the phase velocity is still the same, and the
accelerator structure still has the same number of half wavelengths.
Therefore, the standing wave LINAC cannot be adjusted to cause the
electron bunch to ride at the crest of the electromagnetic wave for
multiple energy levels.

[0109] Traveling wave LINACS have the property that rather than having
discrete modes (as in a standing wave LINAC), they have a continuous pass
band in which the phase velocity (velocity of the electromagnetic wave)
varies continuously with varying frequency. In a TW LINAC the phase
velocity of the electromagnetic wave can be changed with the change in
frequency.

[0110]FIG. 4 illustrates a dispersion curve 34 for an exemplary TW LINAC.
The dispersion curve 34 in FIG. 4 graphs angular frequency
(ω=2πf, wherein f is the frequency of the electromagnetic wave
in the accelerator structure) vs. the propagation constant
(β≡2π/λ, where λ is the wavelength of the
electromagnetic wave in the accelerator structure) for the exemplary TW
LINAC. The propagation constant, β, is the phase shift of the RF
electromagnetic wave per unit distance along the Z axis of the TW LINAC.
The phase velocity of an electromagnetic wave in the TW LINAC is equal to
the slope, ω/β, of the line from the origin to the operating
point, ω,β, which is equal to the frequency times the
wavelength of the electromagnetic wave (fλ). As shown, the phase
velocity of the electromagnetic wave varies continuously with varying
frequency. The group velocity (the velocity with which a pulse of the
electromagnetic wave propagates) is given by dω/dβ, the slope
of the dispersion curve. The change of phase, δφ(z), at a
longitudinal position z in the TW LINAC caused by a change of angular
frequency δφ, is given by the equation:

δφ(z)=δΩ∫dz/(dω/dβ)=δω-
∫dz/vg=δωtf(z) (1)

where tf(z) is the filling time from the beginning of the LINAC to
the position z.

[0111] In general for LINACs, the dispersion curve, and therefore both the
phase velocity and the group velocity, can vary from cell to cell. In the
TW LINAC used as an example here, for the maximum energy operation most
of the LINAC has a constant phase velocity equal to the velocity of
light. However, the structure is designed to have an approximately
constant gradient, which means that the group velocity decreases
approximately linearly with distance along the LINAC. Therefore, when the
frequency is changed (raised) for operation at the lower energy level
(e.g., at 6 MeV), to achieve a maximum possible energy the phase velocity
is no longer constant during the portion of acceleration at which the
electrons travel at approximately the speed of light.

[0112] As the angular frequency of an electromagnetic wave is increased in
the typical, forward wave TW LINAC, the phase velocity of the
electromagnetic wave is decreased. Thus, if the angular frequency of an
electromagnetic wave used to generate a high energy electron beam is
ω1 and the angular frequency of an electromagnetic wave used
to generate a low energy electron beam is ω2, the slope of
ω1/β1 (L1) will be steeper than the slope of
ω2β2 (L2). Accordingly, the phase velocity of the
electromagnetic wave that generates the high energy x-ray beam is higher
than the phase velocity of the electromagnetic wave that generates the
low energy x-ray beam. The angular frequency of the electromagnetic wave
used to generate the high energy x-ray beam can be chosen such that the
phase velocity for the electromagnetic wave (ω1/β1)
is approximately equal to the speed of light, through most of the LINAC.

[0113]FIG. 5 illustrates a dispersion curve 36 for a high efficiency
magnetically coupled reentrant cavity traveling wave LINAC. In the
dispersion curve 36 in FIG. 5, the y-axis represents angular frequency
and the x-axis represents propagation constants. As shown, in the high
efficiency magnetically coupled reentry cavity TW LINAC configuration,
the phase velocity varies continuously with changing frequency. However,
the dispersion curve 36 of FIG. 5 shows a different relationship between
angular frequency and phase velocity than is shown in the dispersion
curve 34 of FIG. 4. For example, in the dispersion curve 36 of FIG. 5,
angular frequency associated with the high energy electron beam is higher
than the angular frequency associated with the low energy electron beam.
This is in contrast to the dispersion curve 34 of FIG. 4, in which the
angular frequency associated with the high energy beam is lower than the
angular frequency associated with the low energy electron beam. The
relationship between angular frequency and phase velocity can differ from
LINAC to LINAC, and therefore the specific angular frequencies that are
used to tune a TW LINAC should be chosen based on the relationship
between angular frequency and phase velocity for the TW LINAC that is
being tuned. A magnetically coupled backward wave traveling wave constant
gradient LINAC with nose cones operating near the 3π/4 or 4π/5 mode
could have a shunt impedance and therefore efficiency as high as a cavity
coupled standing wave accelerator.

[0114] In one embodiment, the phase velocity of the electromagnetic wave
can be adjusted to cause the electron bunch to, on average, travel at the
crest of the electromagnetic wave. Alternately, the phase velocity of the
electromagnetic wave can be adjusted to cause the electron bunch to, on
average, travel ahead of the crest of the electromagnetic wave.
Adjustments to the phase velocity can be achieved for multiple different
energy levels simply by changing the frequency of the electromagnetic
wave to an appropriate level. Such an appropriate level can be determined
based on the dispersion curves as shown in FIGS. 4 and 5. For example,
the RF frequency of the electromagnetic wave can be raised to reduce the
phase velocity of the wave so that the electron bunch moves faster than
the wave and drifts up toward the crest as it travels through the
accelerator. Changing the RF frequency of the TW LINAC is easy to do on a
pulse to pulse basis if the RF source is a klystron 6, thus allowing
interleaving of 2 or more energies at a high repetition rate. Frequency
changes can also be made when other RF sources are used. This strategy
will work for a wide energy range (e.g., including either the full single
structure X-band or the full single structure S-band energy range).

[0115]FIG. 6 illustrates an electron bunch 40 riding an electromagnetic
wave 42 at three different regions in an accelerator structure of a TW
LINAC. FIG. 6 illustrates a lower energy operation of the LINAC. The
electron bunch is depicted in FIG. 6 as substantially non-synchronous.
The phase velocity of the electromagnetic wave has been adjusted such
that the phase velocity is slower than the speed of the electron bunches
(e.g., by increasing the RF frequency of the electromagnetic wave). In
this lower energy beam operation, the electromagnetic fields can be
smaller and the electron beam can be accelerated more slowly in the
buncher region. When the electron bunch leaves the buncher region of the
accelerator structure, it can be behind the crest of the electromagnetic
wave. At approximately the middle of the accelerator structure, the
electron bunch 40 is at the crest of the electromagnetic wave 42. At the
end of the accelerator structure, the electron bunch 40 is ahead of the
crest of the electromagnetic wave 42. On average, the electron bunch 40
is at the crest of the electromagnetic wave 42. Therefore, the electron
bunch has an energy spectrum that is equivalent to an electron bunch that
rides at the crest of a smaller amplitude electromagnetic wave throughout
the accelerator structure. As a result, jitter does not cause a
significant change in energy of the electron beam, and thus of a
resulting x-ray beam.

[0116] In one embodiment, the phase velocity is adjusted so that the bunch
is as far ahead of the crest at the end of the accelerator structure as
it was behind the crest at the end of the buncher region of the
accelerator structure for a given energy level. That way the electrons at
the head of the bunch that gained more energy in the first half of the
accelerator structure than the electrons at the tail of the bunch can
gain less energy in the second half of the accelerator structure, and the
two effects cancel to first order. Similarly, if the RF frequency jitters
by a tiny amount causing the electron bunch to be farther behind at the
beginning so that it gains less energy in the first half of the
accelerator, it gains more energy in the second half, thus minimizing the
energy jitter. The net effect of adjusting the frequency in this way is
to make the energy distribution within the bunch at the end of the
accelerator structure look as if the bunch rode on the crest of a smaller
amplitude wave throughout the accelerator. This adjustment of the
frequency can also maximize the energy gain (provide maximum x-ray yield)
for the particular amplitude of the electromagnetic waves and reduce beam
energy dependence on RF power level.

[0117] In another embodiment, the phase velocity is adjusted so that the
bunch is further ahead of the crest at the end of the accelerator
structure than it was behind the crest at the beginning of the
accelerator structure for a given energy level. In other words, the RF
frequency is raised to above the point where maximum x-ray yield can be
obtained. Such an adjustment can address amplitude jitter introduced into
the accelerating fields of the LINAC based on amplitude jitter in the RF
source. It should be noted, however, that such an adjustment can cause a
wider energy spectrum of the electron beam and the x-rays than adjusting
the phase velocity so that the bunch is as far ahead of the crest at the
end of the accelerator structure as it was behind the crest at the
beginning of the accelerator structure for a given energy level.

[0118] As discussed above, frequency jitter from the RF source,
temperature variation from the accelerator structure and amplitude jitter
from the RF source all cause the electron bunch to move off the peak of
the electromagnetic wave. However, amplitude jitter in the RF source also
causes jitter in the amplitude of the accelerating fields throughout the
LINAC. When the phase velocity (e.g., RF frequency) is adjusted to place
the bunch, on average, ahead of the peak of the electromagnetic wave, the
jitter in the amplitude of the accelerating fields can be ameliorated.
The amplitude of the RF source can also be adjusted to ameliorate the
amplitude jitter. Alternatively, or in addition, the pulse repetition
rate of the LINAC can be changed to ameliorate the sources of jitter. For
example, where there is a 180 Hz or 360 Hz ripple experienced by the TW
LINAC when operating at 6 MeV, the pulse repetition rate can be changed
from 400 pulses per second (pps) to 360 pps to alleviate jitter.

[0119] The jitter in the x-ray yield can be strikingly reduced by raising
the RF frequency above the point where the maximum x-ray yield is
obtained. This is optimum because when the frequency is raised above the
maximum x-ray yield point it reduces the phase velocity of the
electromagnetic wave and moves the bunch ahead of the accelerating crest
on average in the LINAC. Then, if the RF amplitude jitters upward, the
bunch moves farther ahead of the crest and the downward slope of the sine
wave compensates for the increase in the accelerating fields in the
LINAC. At some frequency the derivative of beam energy or x-ray yield
with respect to RF power actually vanishes.

[0120] In one embodiment, the optimum RF frequency depends on the relative
amplitude of the three sources of x-ray yield jitter. If the bunch is
moved forward of the accelerating crest by just increasing the RF
frequency, the beam energy and the x-ray yield will decrease. However,
the bunch can be moved forward of the accelerator crest by increasing
both the frequency and the amplitude of the RF drive, in a manner which
keeps the energy approximately constant. In one embodiment, in the
commissioning of a LINAC system, when a beam energy spectrometer is
available, the function of power versus RF frequency above the maximum
x-ray yield point, for each operating energy, is measured. Then an
operator can find the point along this power versus frequency curve which
gives the best stability and operate there.

[0121] The ability to change the phase velocity of the wave by just
changing the frequency (or by changing the frequency and amplitude)
enables the electron bunch to be at an optimum position relative to an
electromagnetic wave for a given energy level. Therefore, stable x-rays
can be generated at a range of energy levels. This causes the TW LINAC to
be less susceptible to temperature changes, less susceptible to jitter in
the frequency of the electromagnetic wave, and less susceptible to jitter
in the amplitude of the electromagnetic wave.

6.3 Use of a Frequency Controller in the Operation of a Multi-Energy TW
LINAC

[0122] In a multi-energy interleaving operation of a TW LINAC, a frequency
controller can be used to measure the phase shift of the electromagnetic
wave through the LINAC structure by comparing the phase of the
electromagnetic wave at the input of the accelerator structure to the
phase of the electromagnetic wave at the output of the accelerator
structure. The frequency controller can transmit a signal to the
oscillator to modify the frequency of the electromagnetic wave that is
ultimately coupled into the accelerator structure based on the magnitude
of the phase shift detected by the frequency controller. In a
non-limiting example, the frequency controller can be an automatic
frequency controller (AFC). The frequency controller can be a
multi-frequency AFC, and can operate at a set point for each of a number
of different frequencies, with each frequency being associated with each
different energy. The frequency controller can be used to measure the RF
phase of the electromagnetic wave at the output coupler relative to the
RF phase of the electromagnetic wave at the input coupler. With this
information, the frequency controller can be used to adjust the frequency
of the electromagnetic wave, to maintain the phase shift through the
LINAC to a separate set point for each of the different energies of
operation of the LINAC. The frequency controller can facilitate stable
operation with quick settling during rapid switching of a multi-energy
interleaved TW LINAC. For example, the frequency controller can be used
to correct for the effect of rapid thermalization of the TW LINAC
accelerator structure when the system is stepping from standby to full
power, drifts in the temperature of the accelerator structure cooling
water, or drifts in the frequency of the oscillator.

[0123]FIG. 7 shows a block diagram of an embodiment of a TW LINAC
comprising a frequency controller. In the illustration of FIG. 7, the
frequency controller comprises a controller 72 and a phase comparator 74.
In the example of FIG. 7, the phase comparator 74 compares the
electromagnetic wave at the input of the accelerator structure 8 (P1) and
at the output of the accelerator structure 8 (P2) and provides a measure
of the phase shift (ΔP) to the controller 72. The frequency
controller can transmit a signal to the oscillator 76 to tune the
frequency of the oscillator 76. As discussed above, the oscillator 76 can
generate a signal having a frequency that is provided by the frequency
controller, and the RF signal can be amplified by the amplifier 78 and
supplied to a klystron (see FIG. 1A). Thus, the signal from the frequency
controller to the oscillator 76 can ultimately result in a modification
of the frequency of the electromagnetic wave that is coupled into the
accelerator structure, based on the magnitude of the phase shift detected
by the frequency controller. As discussed above, the amplifier 78 may
adjust the RF power supplied to the klystron based on the RF pulse rise
time determined by the intensity controller. The RF pulse rise time may
be calculated at the intensity controller using, for example, a lookup
table. The oscillator 76 can also generate a signal that results in a
change of the frequency of the electromagnetic wave by an amount to
change the operating energy of the LINAC in the time interval between
electromagnetic wave pulses in an interleaving operation. The frequency
controller is illustrated in FIG. 7 as comprising a controller 72 and a
phase comparator 74 as separate units. However, in other embodiments, the
frequency controller can comprise the controller and phase comparator as
an integral unit.

[0124] FIG. 8 shows a block diagram of another embodiment of a TW LINAC
comprising a frequency controller that can be used for a dual energy
operation. In the illustration of FIG. 8, the frequency controller
comprises a controller 82, and two phase comparators (phase comparator A
83 and phase comparator B 84) that are each used for a different energy
of operation of the LINAC. Phase comparator A 83 compares the
electromagnetic wave at the input of the accelerator structure 8 (P1A)
and at the output of the accelerator structure 8 (P2A) and provides a
measure of the phase shift (ΔPA) to the controller 82. Phase
comparator B 84 compares the electromagnetic wave at the input of the
accelerator structure 8 (P1B) and at the output of the accelerator
structure 8 (P2B) and provides a measure of the phase shift (ΔPB)
to the controller 82. The illustration of FIG. 8 includes two oscillators
(oscillator 85 and oscillator 86), each used for a different energy of
operation of the LINAC. Frequency controller 82 can transmit a signal to
oscillator 85 to tune the frequency of oscillator 85 based on the
measured phase shift ΔPA of an electromagnetic wave used to
accelerate a set of electrons to the desired first energy of operation.
In addition, frequency controller 82 can also transmit a signal to
oscillator 86 to tune the frequency of oscillator 86 based on the
measured phase shift ΔPB of an electromagnetic wave used to
accelerate a set of electrons to the desired second energy of operation.
As discussed above, oscillators 85 and 86 can each generate an RF signal
having a frequency that is provided by the frequency controller, and the
RF signal can be amplified by amplifier 88 and supplied to a klystron
(see FIG. 1A). Thus, the signal from the frequency controller to
oscillator 85 (or oscillator 86) can ultimately result in a modification
of the frequency of the electromagnetic wave that is coupled into the
accelerator structure, for a given energy of operation, based on the
magnitude of a phase shift detected by the frequency controller. As
discussed above, the amplifier 88 may adjust the RF power supplied to the
klystron based on the RF pulse rise time determined by the desired energy
of the electron beam. The RF pulse rise time may be calculated using, for
example, a lookup table. The frequency controller is illustrated in FIG.
8 as comprising a controller 82, phase comparator A 83, and phase
comparator B 84 as separate units. However, in other embodiments, the
frequency controller can comprise the controller and the phase
comparators as an integral unit.

[0125]FIG. 9 shows a flow chart of steps in an example operation of the
TW LINAC. In step 90 of FIG. 9, a first electromagnetic wave from an
electromagnetic wave source is coupled into the accelerator structure of
the TW LINAC. In step 91, a first set of electrons having a first pulse
width is injected at the input of the accelerator structure of the TW
LINAC and the first set of electrons is accelerated to a first energy. In
step 92, an intensity adjustment command is received at an intensity
controller which may be external or integrated. In step 93, the intensity
controller determines a pulse width and a beam injection time based on
the command using, for example, a lookup table. In step 94, an RF power
amplitude is determined based on the desired energy of the next pulse. In
step 95, a modified frequency based on a phase shift of the
electromagnetic wave is determined depending on the energy of the next
pulse. A frequency controller may compare the phase of the
electromagnetic wave at the input of the accelerator structure to the
phase of the electromagnetic wave at the output to monitor the phase
shift of the electromagnetic wave. The frequency controller may transmit
a signal to an oscillator that includes a corrected frequency based on
the magnitude of the phase shift detected by the frequency controller.
For example, the corrected frequency can differ from the first frequency
by an amount δf based on magnitude of the phase shift detected (for
example, δf can be a difference on the order of about 0.001 MHz or
more, about 0.01 MHz or more, about 0.03 MHz or more, about 0.05 MHz or
more, about 0.08 MHz or more, about 0.1 MHz or more, or about 0.15 MHz or
more). In step 96, a second electromagnetic wave generated by the
electromagnetic wave source is coupled into the accelerator based on the
modified frequency. An amplifier can cause the electromagnetic wave
source to generate a subsequent electromagnetic wave. As discussed above,
an oscillator can generate a signal having a frequency that is provided
by the frequency controller, and that signal can be amplified by an
amplifier to an RF power based on the determined RF pulse rise time and
supplied to the electromagnetic wave source (such as a klystron). The
electromagnetic wave source can generate the subsequent electromagnetic
wave based on the amplified signal received from the amplifier. In step
97, a second beam of electrons from the electron gun based on the
determined pulse width is injected at the determined beam injection time,
wherein the second beam of electrons is accelerated by the second
electromagnetic wave to a second range of energies. Advantageously, the
central value of the second range of energies may be substantially the
same as a central value of the first range of energies in a single energy
operation. The range of output energies of two different sets of
electrons is substantially the same if the central value (e.g., the mean
value or median value) of the range of output energies differs by less
than about 0.1%, less than about 1%, less that about 2%, or more. Steps
90-96 can be repeated a number of times during operation of the TW LINAC.

[0126] In an interleaving operation, the LINAC can be operated to cycle
between two different output energies while the x-ray intensity is
modulated from pulse-to-pulse. For example, the LINAC can be operated to
alternate between about 6 MeV and about 9 MeV. In such an operation,
after step 95 but prior to step 96, the LINAC can be operated at an
energy (for example, about 9 MeV) that is different from the first energy
of the first set of electrons (for example, about 6 MeV). The amplitude
and frequency in the accelerator structure of the electromagnetic wave
used for accelerating these additional electrons can be different than
the electromagnetic wave used in step 90. For example, in the
interleaving operation, a first electromagnetic wave is generated and
used to accelerate a first set of electrons having a first pulse width to
the first energy, a second electromagnetic wave (of a different amplitude
and frequency) is generated and used to accelerate a second set of
electrons having a second pulse width, based on a first intensity
adjustment command, that is different from the first pulse width to a
second energy that is different from the first energy. Then, a subsequent
electromagnetic wave is generated based on the phase shift of the first
electromagnetic wave (as discussed above) and used to accelerate a
subsequent set of electrons having a third pulse width, based on a second
intensity adjustment command, different from the first and second pulse
width to substantially the same range of energies as the first energy.
Then, a subsequent electromagnetic wave is generated based on the phase
shift of the second electromagnetic wave and used to accelerate a
subsequent set of electrons having a fourth pulse width, based on a third
intensity adjustment command, different from the first, second, and third
pulse width to substantially the same range of energies as the second
energy, and so on. Although this interleaving operation is described as a
dual energy interleaving operation, it should be noted that the exemplary
TW LINAC is not limited thereto.

[0127] In yet another example of an interleaving operation, the LINAC is
operated for multiple pulses at the first energy before it is operated at
the second energy. The LINAC can also be operated to provide multiple
pulses at the first energy and then operated to provide multiple pulses
at the second energy.

[0128] In another example operation, prior to step 90, a phase set point
for the first energy can be input into the phase comparator. The phase
shift can be inserted into one input arm of the phase comparator so that
the phase comparator outputs a reading of, e.g., zero voltage, when the
phase is correct for the desired energy of the pulse. In another example,
after step 92 and prior to step 94, a phase set point for the second
energy can be input into the phase comparator.

[0129] The frequency controller can have several different set points for
the optimum phase shift for each of the different energies at which the
TW LINAC is operated. For example, the frequency controller can have N
different set points for the optimum phase shift that corresponds to each
of N different energies (N≧2) at which the TW LINAC is operated.

[0130] The frequency controller can perform the phase comparison
continuously as a beam of electrons is accelerated in the accelerator
structure. For example, the frequency controller can perform the phase
comparison continuously from the moment an electromagnetic wave reaches
the output end of the accelerator structure until the end of the RF pulse
reaches the input of the accelerator structure. The set point for the
phase bridge can be changed before another electromagnetic wave is
coupled into the accelerator structure, so that the set point is
appropriate for the intended energy range of the subsequent pulse of
output electrons.

[0131] The frequency controller can adjust the frequency to achieve the
desired phase set point. For example, for a TW LINAC in which the
accelerator structure is a forward wave structure, the frequency
controller can transmit a signal to result in the raising of the
frequency for the lower energy operation in which the electron beam is
moving slower through the buncher region. In another example, for a TW
LINAC in which the accelerator structure is a forward wave structure, the
frequency controller can transmit a signal to result in the lowering of
the frequency for the higher energy operation in which the electron beam
is moving faster through the buncher region. The transit time of the
electron beam through the buncher region can differ greatly from the
lower energy operation to the higher energy operation when the electrons
are being accelerated from, e.g., about 15 keV (an example energy of
electrons emerging from an electron gun) to about 1 MeV. The difference
in transit times results from the different electric field amplitudes
being applied to the electrons for the lower energy beam versus the
higher energy beam. For example, electric field amplitudes used for the
lower energy beam can be about 2/3 as high as that used for the higher
energy beam in a dual-energy operation. The frequency controller can
transmit a signal to result in the adjustment of the frequency of the
electromagnetic wave to make the transit time of the electromagnetic wave
crests through the structure optimized for the transit time of the
electrons through the accelerator structure for each of the different
energies in the interleaved operation of the TW LINAC. For example,
frequency controller can transmit a signal to provide electromagnetic
wave crests whose transit time through the accelerator structure is
longer for lower energy electron beams.

[0132] In examples where the accelerator structure is a backward wave
structure, the sign of the frequency change in the foregoing discussions
would be reversed. For example, if the frequency is raised to achieve a
result for a forward wave structure, it is lowered to achieve that result
for a backward wave structure.

[0133] Changing the frequency of the electromagnetic wave can change the
phase velocity of the wave so that, at each electron beam energy, the
electron bunch can be on the average on the crest of the wave. The TW
LINAC can be configured so that, for one particular energy, termed the
synchronous energy, the buncher region and the accelerating structure of
the LINAC can be designed so that the bunch is near the crest all the way
through the LINAC. If the TW LINAC is to be operated over a large energy
range, e.g., energies ranging from 3 MeV to 9 MeV, the synchronous energy
can be chosen to be near the middle of the operating range.

[0134] If the input power (and hence amplitude) of the electromagnetic
wave is lowered to lower the fields, and thus lower the energy of the
electron beam, the fields can decrease uniformly throughout the LINAC.
However, the effect of the decrease in power of the electromagnetic wave
(including decreased electron velocity) can be more concentrated in the
buncher region, since the velocity of the electrons becomes considerably
less sensitive to the power of the electromagnetic wave once the
electrons approach relativistic speeds. A change in phase velocity of the
wave resulting from a change in frequency for a constant gradient forward
wave TW LINAC can be small at the input end of the accelerator structure
and large at the output end. The frequency controller can transmit a
signal to change the frequency of an electromagnetic wave such that the
electron bunch travels substantially behind the crest in the first third
of the accelerator structure, to reach the crest by around the middle of
the accelerator structure, and to be substantially ahead of the crest in
the last third of the accelerator structure. In this example, the energy
correlation as a function of position within the electron bunch that the
electrons gain in the first third of their travel through the LINAC can
be removed by traveling ahead of the crest in the last third of their
travel through the LINAC. The frequency adjustment that removes the
energy correlation as a function of position can also maximize the energy
gain through the LINAC, and can maximize the x-ray yield.

[0135] For a given energy of operation, the optimum frequency and the set
point of the frequency controller can be functions of both the energy and
the beam current from the electron gun. The beam current from the
electron gun can be varied to change the output energy of the electrons
through the beam loading effect. In the beam loading effect, the electron
beam bunched at the operating frequency of the LINAC can induce a field
in the accelerator structure that has a phase that opposes the
acceleration applied by the electromagnetic wave coupled into the LINAC,
and can act to oppose the forward motion of the electrons. That is, beam
loading can induce fields that act to decelerate the electron beam. The
amplitude of these induced fields vary linearly with the magnitude of the
beam current, and can rise roughly linearly with distance along the
accelerator structure. A higher electron beam current can induce electric
fields of higher amplitude that oppose the acceleration applied by the
electromagnetic wave coupled into the LINAC, and result in the electron
beam experiencing less acceleration. In effect, beam loading can decrease
the amplitude of the electromagnetic wave. A desirable result of
increasing the electron gun current (and hence the effect of beam
loading) to lower the energy of the output electrons can be that the
x-ray yield can be increased, for example, from the increased dose rate
of electrons.

[0136] The beam loading effect can lower the energy of the electron beam,
while having little effect on the transit time of the electron beam
through the accelerator, since the electron beam induced fields are small
at the input end where the electron beam is non-relativistic. If the
power of the electromagnetic wave is raised in an effort to compensate
for the lowered energy that can result from beam loading, the fields can
change equally in all cavities of the accelerator structure and have a
strong effect on the beam transit time through the accelerator structure.
Thus, for each different energy in an interleaving operation, an
adjustment in the set point of the frequency controller can be made to
account for the different RF phase shifts through the LINAC that can
occur for each different energy of operation, for example, due to the
effect of beam loading.

[0137] In a multi-energy operation of the LINAC, the electron gun can be
operated at a different beam current for each energy of operation. As
discussed above, increasing the beam current for the lower energy
operation can provide an increased x-ray yield at the lower energy than
achieved by just lowering the amplitude of the electromagnetic wave from
the klystron. Using a different beam current from the electron gun for
each different energy of operation of the LINAC can help maintain the
same x-ray intensity across the different energies of operation.

[0138] In another embodiment, an operator can choose a phase shift through
the LINAC for each different energy which maximizes the x-ray yield for
that energy. That is, an operator can choose the set point of the
frequency controller for each different energy of operation. The
frequency controller can then continuously adjust the frequency of the
electromagnetic wave to maintain the phase of the electromagnetic wave at
the preset phase set point for that energy. It has been demonstrated that
a similar value of phase shift through the LINAC can optimize the
electron spectrum (i.e., eliminate the energy correlation with position
in the bunch along the longitudinal direction of the LINAC), maximize the
energy, and maximize the x-ray yield. However, maximizing the x-ray yield
can be sensitive to frequency and can be easy to perform.

[0139] In an embodiment, the frequency controller can maintain automatic
control over the adjustments to the frequency of the electromagnetic wave
in a feedback operation. In a non-limiting example, the frequency
controller can be an automatic frequency controller (AFC).

[0140] In another embodiment, a frequency controller can maintain
automatic control and adjust the frequency of the electromagnetic wave to
stabilize the energy of the electrons output at a given energy of
operation. The energy of the electrons are stabilized when the energy
spectrum of the electrons is centered at or substantially near the
desired energy of operation of the accelerator (i.e., the maximum
attainable energy of the LINAC for the given electromagnetic fields), and
the full-width at half-maximum of the energy spectrum of the output
electrons is minimized (i.e., narrowed). All of the systems and methods
disclosed herein are also applicable to this embodiment of the operation
of the TW LINAC comprising the frequency controller. For example, the
frequency controller can maintain automatic control and adjust the
frequency of the electromagnetic wave to stabilize the energy of the
electrons at each energy of operation. In this example, the frequency
controller can compare a first output of electrons at an energy to a
second output of electrons at that same energy, and frequency controller
transmits a signal to an oscillator, and adjust the frequency of the
electromagnetic wave to stabilize the output of electrons. The frequency
of the electromagnetic wave can be varied on alternate pulses of the same
energy to determine the behavior of the measured output of electrons
versus frequency, and thus determine the change in frequency that can
cause the output of electrons to peak around the desired energy, with
minimized energy spread.

[0141] In another embodiment, the frequency controller can maintain
automatic control and adjust the frequency of the electromagnetic wave to
maximize the yield of x-rays at each energy (generated by contacting a
target with the output electrons). For example, the frequency controller
can transmit a signal to adjust the frequency of the electromagnetic wave
based on the measured yield of x-rays. The maximum of the yield of x-rays
at a given energy of the interleaving operation can be predetermined. The
frequency of the electromagnetic wave can be varied on alternate pulses
of the same energy to determine the behavior of the measured yield of
x-rays versus frequency, and thus determine the change in frequency that
can cause the yield to move towards the maximum. In this example, the
yield of x-rays on two successive pulses at the same energy can be
compared to determine the adjustment to the electromagnetic wave
frequency. In a specific embodiment, the frequency can be varied by about
100 kHz on alternate pulses of the same energy, resulting in a change in
phase through the structure of about 8 degrees of phase. With this
frequency variation, the electron bunch can alternate between about 2
degrees forward and about 2 degrees behind the crest of the
electromagnetic wave on successive pulses of the same energy.

[0142] The frequency controller can maintain automatic control over the
adjustments to the frequency of the electromagnetic wave in a feedback
operation. A feedback loop can be intricate and the convergence time to
determine a frequency adjustment can be long. The convergence time can be
reduced by making the frequency correction (or adjustment) proportional
to the error signal. In the embodiment where the frequency controller is
used to maximize the yield of x-rays at each energy of operation, the
error signal can be determined as the difference between the x-ray yield
from two pulses, divided by the sum of the x-ray yields from the two
pulses. The energy of the beam can be approximated as a sine function of
phase shift through the LINAC. Normalizing by the sum of the two x-ray
yields can cause the error signal measure to be insensitive to changes in
the x-ray measurement device. In the embodiment where the frequency
controller is used to stabilize the energy of the output electrons at
each energy of operation, the error signal can be determined as the
difference between the electron current from two pulses, divided by the
sum of the electron currents from the two pulses.

[0143] A frequency controller operated in a feedback operation can be used
to correct for the effect of minor drifts of the electron gun current or
minor drifts of the RF power (hence amplitude). That is, in addition to
correcting for drifts in the temperature of the accelerator structure or
drifts in the frequency of the oscillator.

6.4 X-Rays

[0144] In certain aspects, x-rays can be generated from the bombardment of
a target material by the accelerated electron beam or electron bunches
from a LINAC. The x-rays can be generated by two different mechanisms. In
the first mechanisms, collision of the electrons from the LINAC an atom
of a target can impart enough energy so that electrons from the atom's
lower energy levels (inner shell) escape the atom, leaving vacancies in
the lower energy levels. Electrons in the higher energy levels of the
atom descend to the lower energy level to fill the vacancies, and emit
their excess energy as x-ray photons. Since the energy difference between
the higher energy level and the lower energy level is a discrete value,
these x-ray photons (generally referred to as k-shell radiation) appear
in the x-ray spectrum as sharp lines (called characteristic lines).
K-shell radiation has a signature energy that depends on the target
material. In the second mechanisms, the electron beams or bunches from
the LINAC are scattered by the strong electric field near the atoms of
the target and give off Bremsstrahlung radiation. Bremsstrahlung
radiation produces x-rays photons in a continuous spectrum, where the
intensity of the x-rays increases from zero at the energy of the incident
electrons. That is, the highest energy x-ray that can be produced by the
electrons from a LINAC is the highest energy of the electrons when they
are emitted from the LINAC. The Bremsstrahlung radiation can be of more
interest than the characteristic lines for many applications.

[0145] Materials useful as targets for generating x-rays include tungsten,
certain tungsten alloys (such as but not limited to tungsten carbide, or
tungsten (95%)-rhenium (5%)), molybdenum, copper, platinum and cobalt.

6.5 Instrumentation

[0146] Certain instruments which may be used in the operation of a
traveling wave LINAC include a klystron modulator and an electromagnetic
wave source.

[0147] 6.5.1 Modulator

[0148] A modulator generates high-voltage pulses lasting a few
microseconds. These high-voltage pulses can be supplied to the
electromagnetic wave source (discussed in Section 6.5.2 below), to the
electron gun (see Section 6.1 above), or to both simultaneously. A power
supply provides DC voltage to the modulator, which converts this to the
high-voltage pulses. For example, the Solid State Klystron Modulator -K1
or -K2 (ScandiNova Systems AB, Uppsala, Sweden) can be used in connection
with a klystron.

[0149] 6.5.2 Microwave Generators

[0150] The electromagnetic wave source can be any electromagnetic wave
source deemed suitable by one of skill. The electromagnetic wave source
(in the microwave of radio frequency ("RF") range) for the LINAC can be a
klystron amplifier (discussed in Section 6.1 above). In a klystron, the
size of the RF source and the power output capability are roughly
proportional to the wavelength of the electromagnetic wave. The
electromagnetic wave can be modified by changing its amplitude,
frequency, or phase.

6.6 Exemplary Apparatus and Computer-Program Implementations

[0151] Aspects of the methods disclosed herein can be performed using a
computer system, such as the computer system described in this section,
according to the following programs and methods. For example, such a
computer system can store and issue commands to facilitate modification
of the electromagnetic wave frequency and power according to a method
disclosed herein. In another example, a computer system can store and
issue commands to facilitate operation of the frequency controller
according to a method disclosed herein. The systems and methods may be
implemented on various types of computer architectures, such as for
example on a single general purpose computer, or a parallel processing
computer system, or a workstation, or on a networked system (e.g., a
client-server configuration such as shown in FIG. 10).

[0152] An exemplary computer system suitable for implementing the methods
disclosed herein is illustrated in FIG. 10. As shown in FIG. 10, the
computer system to implement one or more methods and systems disclosed
herein can be linked to a network link which can be, e.g., part of a
local area network ("LAN") to other, local computer systems and/or part
of a wide area network ("WAN"), such as the Internet, that is connected
to other, remote computer systems. A software component can include
programs that cause one or more processors to issue commands to one or
more control units, which cause the one or more control units to issue
commands to cause the initiation of the frequency controller, to cause
the initiation of the intensity controller, to operate the
electromagnetic wave source to generate an electromagnetic wave at a
frequency, and/or to operate the LINAC (including commands for coupling
the electromagnetic wave into the LINAC). The programs can cause the
system to retrieve commands for executing the steps of the methods in
specified sequences, including initiating the frequency controller,
computing the RF power(s), and operating the electromagnetic wave source
to generate an electromagnetic wave at a frequency and at an RF power,
from a data store (e.g., a database). The data store may be configured to
store beam parameters such as the gun current, RF power, RF frequency,
AFC phase set point, gun pulse length, gun timing, RF pulse length, and
RF pulse timing for each electron beam. For example, for a 3-energy LINAC
with 6 different intensities for each of the 3 energies, the data store
would store the beam parameters for each of the 18 different beams (3
energies times 6 intensities). Such a data store can be stored on a mass
storage (e.g., a hard drive) or other computer readable medium and loaded
into the memory of the computer, or the data store can be accessed by the
computer system by means of the network.

[0153] In addition to the exemplary program structures and computer
systems described herein, other, alternative program structures and
computer systems will be readily apparent to the skilled artisan. Such
alternative systems, which do not depart from the above described
computer system and programs structures either in spirit or in scope, are
therefore intended to be comprehended within the accompanying claims.

[0154] 6.7 Systems for Cargo Scanning

[0155] As noted above, the TW LINAC of the present invention suitably may
be used in a variety of systems and methods, including systems and
methods for cargo scanning or radiotherapy. An exemplary system for
scanning cargo, and exemplary method for using the same, is described in
this section with reference to FIGS. 11 and 12. An exemplary system for
radiotherapy, and an exemplary method for using the same, is described
further below in section 6.8 with reference to FIGS. 13 and 14.

[0156] Referring to FIG. 11, an illustrative system 1100 for scanning
cargo includes TW LINAC 1110, which may be substantially the same as the
TW LINAC described above and which includes intensity controller 13
configured to adjust various parameters of the TW LINAC so as to provide
an output dose rate and energy of electrons. System 1100 also includes
control unit 1120, target 22 which may be substantially the same as
target 22 described above, and x-ray detector 1130.

[0157] As described above, TW LINAC 1110 is configured to generate
electron beams having selected doses and/or energies. The electron beams
from TW LINAC 1110 travel along electron beam path 1151 and irradiate
x-ray target 22, which may be considered to be part of TW LINAC 1110, and
in the illustrative embodiment is copper (Cu). X-ray target 22 is
configured to generate x-rays responsive to irradiation with electrons.
The resulting x-rays, which also have selected doses and/or energies
based on the particular configuration of TW LINAC 1110, travel along
x-ray beam path 1152 and irradiate object 1140, which may be a cargo
container, for example. The x-rays become attenuated as they travel
through object 1140, based on the particular materials and thicknesses of
materials in object 1140, and then travel along attenuated x-ray beam
path 1153. X-ray detector 1130 is configured to detect the x-rays
transmitted through object 1140.

[0158] Control unit 1120 is in operative communication with detector 1130
and with intensity controller 13 of TW LINAC 1110, and is configured to
generate a plurality of intensity/energy adjustment commands
corresponding to different desired combinations of output dose rates and
energies. Each such intensity/energy adjustment command causes intensity
controller 13 to determine a corresponding pulse width, beam injection
time, and frequency adjustment factor to provide the respective desired
output dose rate and energy of electrons. For each such intensity/energy
adjustment command, intensity controller 13 then issues appropriate
commands to the electron gun modulator 9, amplifier 3, and frequency
controller 1 in the manner described above. Such commands cause TW LINAC
1110 to generate electrons having the respective output dose rate and
energy. Control unit 1120 may issue such commands based on the percent
transmission of x-rays through object 1140, as described in further
detail below.

[0159] Specifically, control unit 1120 may be configured to cause TW LINAC
1110 to generate sequences of electron output dose rates and energies
that are particularly well suited for use in cargo scanning operations,
e.g., in which the electron energies and doses are selected so as to
obtain sufficient transmission of x-rays through an object to
sufficiently interrogate the object. Some of such sequences may be
considered to constitute "dynamic intensity and energy variation," in
that the energy and/or dose of the electrons may be increased or
decreased as rapidly as on a pulse-to-pulse basis.

[0160] In some embodiments, the energy of the x-rays may be held constant,
and the dose varied so as to provide increasing or decreasing amounts of
x-rays through object 1140. For example, in circumstances where a first
dose of x-rays at a given energy results in a percent transmission
through object 1140 that is below a predetermined threshold, e.g., is
insufficient to interrogate object 1140, the energy of the x-rays may be
held constant and a second, increased dose of x-rays generated by issuing
appropriate commands from control unit 1120 to intensity modulator 13.
Such an increase may be particularly useful in that a greater dose of
x-rays may increase the amount of information obtained about the object.
Or, for example, in circumstances where a first dose of x-rays at a given
energy results in a percent transmission through object 1140 that is
above a predetermined threshold, e.g., is greater than required to
interrogate object 1140, the energy of the x-rays may be held constant
and a second, decreased dose of x-rays generated by issuing appropriate
commands from control unit 1120 to intensity modulator 13. Such a
decrease may be particularly useful in that lowering the dose of x-rays
may reduce operator exposure to the x-rays.

[0161] In other embodiments, the dose of the x-rays may be held constant,
and the energy varied so as to provide x-rays having increasing or
decreasing energies through object 1140. It may be useful to hold the
x-ray dose constant while varying the energy, because it allows images
obtained at different energies to be directly compared to one another to
characterize the object. For example, in circumstances where x-rays
having a first energy at a given dose results in a percent transmission
through object 1140 that is below a predetermined threshold, e.g., is
insufficient to interrogate object 1140, the dose of the x-rays may be
held constant and x-rays having a second, increased energy generated by
issuing appropriate commands from control unit 1120 to intensity
modulator 13. Such an increase may be particularly useful in that higher
energy x-rays may better penetrate object 1140 and thus increase the
amount of information obtained about the object. Or, for example, in
circumstances where x-rays having a first energy at a given dose results
in a percent transmission through object 1140 that exceeds a
predetermined threshold e.g., is greater than required to interrogate
object 1140, the dose of the x-rays may be held constant and x-rays
having a second, decreased energy generated by issuing appropriate
commands from control unit to intensity modulator 13. Such a decrease may
be particularly useful in that lowering the energy of x-rays may reduce
operator exposure to the x-rays. The energy of the electrons may be
varied between several discrete energies, e.g., between 4 MeV, 6 MeV, and
9 MeV, or may be varied along a spectrum of possible energies, e.g., at
any energy between 3 MeV and 9 MeV. Additionally, even if the x-ray dose
is held constant, the corresponding output dose rate of electrons need
not necessarily be constant, because different energies of electrons may
generate x-rays with different yields. As such, a higher electron energy
may more efficiently convert to x-rays than would a lower electron
energy, so a lower output dose rate of electrons at the higher energy may
be used to provide the same dose of x-rays as at the lower energy.

[0162] In still other embodiments, both the output dose rate and the
energy of the electron beam may be dynamically varied so as to generate
x-rays having dynamic intensity and energy variation, so as to
sufficiently interrogate object 1140 while reducing operator exposure to
x-rays. For example, as illustrated in FIG. 12, method 1200 includes
irradiating an object with x-rays having a first energy and a selected
dose (step 1210). For example, control unit 1120 may send a first
intensity adjustment command to cause intensity controller 13 to
determine a first pulse width, a first beam injection time, and a first
frequency adjustment factor to provide a first output dose rate and first
energy of a first set of electrons. A first set of x-rays generated by
those electrons then pass through object 1140 and are detected by
detector 1130. The dose and energy of the first set of x-rays is based on
the first output dose rate and first energy of the first set of
electrons.

[0163] Method 1200 also includes determining whether the percent
transmission of the x-rays through the object is below a first
predetermined threshold (step 1220). For example, control unit 1120 may
obtain the percent transmission of the x-rays based on an output of
detector 1130, and optionally also based on an output from x-ray yield
monitor 31 described further above with reference to FIG. 2. Control unit
1120 may compare the percent transmission to the first predetermined
threshold, which may be a value representative of a minimum dose of
attenuated x-rays required to obtain a sufficiently clear image of object
1140.

[0164] If the percent transmission is below the first predetermined
threshold, method 1200 also includes irradiating the object with a second
set of x-rays having a second energy, which may be higher than the first
energy, and with the same dose of x-rays as in step 1210 (step 1230). For
example, control unit 1120 may send a second intensity adjustment command
to cause intensity controller 13 to determine a second pulse width, a
second beam injection time, and a second frequency adjustment factor to
provide a second output dose rate and second energy of a second set of
electrons. A second set of x-rays generated by those electrons then pass
through object 1140 and are detected by detector 1130. The dose and
energy of the second set of x-rays is based on the second output dose
rate and second energy of the second set of electrons. As noted above,
although the dose of the second set of x-rays may be the same as that of
the first set of x-rays, the output dose rate of the second set of
electrons may be different from the output dose rate of the first set of
electrons, depending on the degree to which the conversion efficiency of
electrons to x-rays varies with electron energy. The intensity controller
may be configured to select the second output dose rate of the second set
of electrons such that a dose of the second set of x-rays is
substantially the same as the dose of the first set of x-rays.

[0165] Method 1200 also includes determining whether the percent
transmission of the x-rays of step 1230 through the object is below a
first predetermined threshold (step 1240). For example, control unit 1120
may obtain the percent transmission of the x-rays of step 1230 based on
an output of detector 1130, and optionally also based on an output from
x- ray yield monitor 31 described further above with reference to FIG. 2.
Control unit 1120 may compare the percent transmission to the first
predetermined threshold, which may be a value representative of a minimum
dose of attenuated x-rays required to obtain a sufficiently clear image
of object 1140.

[0166] If the percent transmission is below the first predetermined
threshold, method 1200 also includes irradiating the object with a third
set of x-rays having a third energy, which may be higher than the second
energy, and with the same dose of x-rays as in steps 1210 and 1230 (step
1250). For example, control unit 1120 may send a third intensity
adjustment command to cause intensity controller 13 to determine a third
pulse width, a third beam injection time, and a third frequency
adjustment factor to provide a third output dose rate and third energy of
a first set of electrons. A third set of x-rays generated by those
electrons then pass through object 1140 and are detected by detector
1130. The dose and energy of the third set of x-rays is based on the
third output dose rate and third energy of the first set of electrons.
The third set of x-rays may have a higher energy than the second set of
x-rays. However, although the dose of the third set of x-rays may be the
same as that of the first and second sets of x-rays, the output dose rate
of the third set of electrons may be different from the output dose rate
of the first and/or set of electrons, depending on the degree to which
the conversion efficiency of electrons to x-rays varies with electron
energy. The intensity controller may be configured to select the third
output dose rate of the third set of electrons such that a dose of the
third set of x-rays is substantially the same as the doses of the first
and second sets of x-rays

[0167] Method 1200 also includes determining whether the percent
transmission of the x-rays of step 1250 through the object is below a
first predetermined threshold (step 1260). For example, control unit 1120
may obtain the percent transmission of the x-rays of step 1230 based on
an output of detector 1130, and optionally also based on an output from
x-ray yield monitor 31 described further above with reference to FIG. 2.
Control unit 1120 may compare the percent transmission to the first
predetermined threshold, which may be a value representative of a minimum
dose of attenuated x-rays required to obtain a sufficiently clear image
of object 1140.

[0168] If the percent transmission is below the first predetermined
threshold, method 1200 also includes increasing the selected dose, and
then repeating the same energy sequence as set forth in steps 1210
through 1260 (step 1270). As such, the next energy generated may be lower
than the third energy, but the next dose may be higher than the third
dose.

[0169] Method 1200 also includes determining whether the percent
transmission of the x-rays of step 1210 through the object is above a
second predetermined threshold (step 1221). For example, control unit
1120 may obtain the percent transmission of the x-rays based on an output
of detector 1130, and optionally also based on an output from x-ray yield
monitor 31 described further above with reference to FIG. 2. Control unit
1120 may compare the percent transmission to the second predetermined
threshold, which may be a value representative of a maximum dose of
attenuated x-rays required to obtain a sufficiently clear image of object
1140.

[0170] If the percent transmission is above the second predetermined
threshold, method 1200 includes decreasing the dose and/or energy of the
next set of x-rays generated (step 1222). For example, control unit 1120
may send a second intensity adjustment command to cause intensity
controller 13 to determine a second pulse width, a second beam injection
time, and a second frequency adjustment factor to provide a second output
dose rate and second energy of a second set of electrons. A second set of
x-rays generated by those electrons then pass through object 1140 and are
detected by detector 1130. The dose and energy of the second set of
x-rays is based on the second output dose rate and second energy of the
second set of electrons. The intensity controller may be configured to
select the second output dose rate of the second set of electrons such
that a dose of the second set of x-rays is less than the dose of the
first set of x-rays.

[0171] It will be appreciated that sequences of doses and energies other
than those described above may readily be employed for scanning cargo, or
for other purposes.

[0172] 6.8 Systems for Radiotherapy

[0173] An exemplary system for radiotherapy, and an exemplary method for
using the same, will now be described with reference to FIGS. 13 and 14.

[0174] Referring to FIG. 13, an illustrative system 1300 for radiotherapy
includes TW LINAC 1310, which may be substantially the same as TW LINAC
1110 described above and which includes intensity controller 13
configured to adjust various parameters of the TW LINAC so as to provide
an output dose rate and energy of electrons. System 1300 also includes
control unit 1320, target 22 which may be substantially the same as
target 22 described above, and robotic arm 1330 on stand 1331.

[0175] As described above, TW LINAC 1310 is configured to generate
electron beams having selected doses and/or energies. The electron beams
from TW LINAC 1310 travel along electron beam path 1351 and irradiate
x-ray target 22, which may be considered to be part of TW LINAC 1310, and
in the illustrative embodiment is copper (Cu). X-ray target 22 is
configured to generate x-rays responsive to irradiation with electrons.
The resulting x-rays, which also have selected doses and/or energies
based on the particular configuration of TW LINAC 1310, travel along
x-ray beam path 1352 and irradiate tumor volume 1340, which may be a
cancer tumor, for example. Robotic arm 1330 is configured to modify the
angle at which the x-rays irradiate tumor volume 1340. As illustrated in
FIG. 13, the x-rays may be relatively narrowly collimated so as to have a
relatively high intensity within tumor volume 1340, so as to cause
necrosis (tissue death) within the volume when the volume is repeatedly
irradiated from multiple angles under control of robotic arm 1330, but
preferably without significantly damaging the surrounding tissue.

[0176] Control unit 1320 is in operative communication with robotic arm
1330 and with intensity controller 13 of TW LINAC 1310, and is configured
to generate a plurality of intensity/energy adjustment commands
corresponding to different desired combinations of output dose rates and
energies. Each such intensity/energy adjustment command causes intensity
controller 13 to determine a corresponding pulse width, beam injection
time, and frequency adjustment factor to provide the respective desired
output dose rate and energy of electrons. For each such intensity/energy
adjustment command, intensity controller 13 then issues appropriate
commands to the electron gun modulator 9, amplifier 3, and frequency
controller 1 in the manner described above. Such commands cause TW LINAC
1310 to generate electrons having the respective output dose rate and
energy. Control unit 1320 also is configured to issue commands causing
robotic arm 1330 to adjust the angle at which the x- rays irradiate tumor
volume 1340. Control unit 1320 may issue such commands to intensity
controller 13 and to robotic arm 1330 so as to achieve substantially
homogenous irradiation of tumor volume 1340, as described in further
detail below.

[0177] Specifically, control unit 1320 may be configured to cause TW LINAC
1310 to generate sequences of electron output dose rates and energies
that are particularly well suited for use in radiotherapy operations,
e.g., in which the electron energies and doses are selected so as to more
homogenously irradiate tumor volume 1340 when combined with angular
control over the irradiation by robotic arm 1330. Some of such sequences
may be considered to constitute "dynamic intensity and energy variation,"
in that the energy and/or dose of the electrons may be increased or
decreased as rapidly as on a pulse-to-pulse basis.

[0178] For example, at a given angle of irradiation and a given dose, the
energies of x-rays may be controlled so as to irradiate tumor volume 1340
at varying depths. For example, x-rays having a low energy may penetrate
tumor volume 1340 to a relatively low extent, and thus primarily may be
used to irradiate more superficial regions of tumor volume 1340. In
comparison, x-rays having a higher energy may penetrate tumor volume 1340
to a greater extent, and thus primarily may be used to irradiate deeper
regions of tumor volume 1340. However, the higher and lower energies of
x-rays both may irradiate the same portion of tumor volume 1340, albeit
with different respective doses than one another in any given section of
that portion. As such, the doses and energies of all x-ray beams that
irradiate a given section determine the composite dose received by that
section at a given angle. The angles may be varied to irradiate different
portions of the tumor so as to fully treat the tumor. According to the
present invention, the doses, energies, and angles of the x-rays
preferably are selected so that each section of the tumor volume 1340
receives a desired dose of x-rays, which in some embodiments is
substantially the same dose of radiation as each other section of the
tumor volume.

[0179] For example, method 1400 illustrated in FIG. 14 illustrates method
1400 that may be used with system 1300 of FIG. 13 in a radiotherapy
operation. Method 1300 includes irradiating a tumor volume with x-rays
having a first energy and a first dose, from a first angle (step 1410).
For example, control unit 1320 may send a first intensity adjustment
command to cause intensity controller 13 to determine a first pulse
width, a first beam injection time, and a first frequency adjustment
factor to provide a first output dose rate and first energy of a first
set of electrons. Control unit 1320 also may send a first position
command to robotic arm 1330 to cause the robotic arm to adjust the angle
of irradiation, such that a first portion of the tumor volume is
irradiated with x-rays generated by the first set of electrons. The dose
and energy of the first set of x-rays is based on the first output dose
rate and first energy of the first set of electrons.

[0180] Method 1400 also includes irradiating a tumor volume with x-rays
having a second energy and a second dose, from a second angle (step
1420). For example, control unit 1320 may send a second intensity
adjustment command to cause intensity controller 13 to determine a second
electron pulse width, a second beam injection time, and a second
frequency adjustment factor to provide a second output dose rate and
second energy of a second set of electrons. Control unit 1320 also may
send a second position command to robotic arm 1330 to cause the robotic
arm to adjust the angle of irradiation, such that a second portion of the
tumor volume is irradiated with x-rays generated by the second set of
electrons. The dose and energy of the second set of x-rays is based on
the second output dose rate and second energy of the second set of
electrons. The energy of the second set of x-rays may be the same as,
higher than, or lower than, the energy of the first set of x-rays.
Similarly, the dose of the second set of x-rays may be the same as,
higher than, or lower than, the energy of the first set of x-rays.
Further, the angle of the second set of x-rays may be the same as, or
different than, the angle of the first set of x-rays. However, at least
one of these three parameters (energy, dose, and angle) is different
between the first and second sets of x-rays, and each of the parameters
is selected such that the tumor volume is irradiated with a desired
composite dose (e.g., a homogeneous dose).

[0181] Steps 1410 and 1420 are repeated for different portions of the
tumor volume, until the entire volume is irradiated with x-rays (step
1430). In one preferred embodiment, the energies, doses, and angles are
selected such that each portion of the tumor volume receives
substantially the same dose of x-rays as each other portion.

7. MODIFICATIONS

[0182] Many modifications and variations of this invention can be made
without departing from its spirit and scope, as will be apparent to those
skilled in the art. For example, other types of linear accelerators
suitably may be used to generate X-ray energy and dose sequences
analogous to those described herein. The specific embodiments described
herein are offered by way of example only, and the invention is to be
limited only by the terms of the appended claims, along with the full
scope of equivalents to which such claims are entitled.